5 Tax Mistakes Small Business Owners Make in Q4 (And How to Avoid Them)

By Vivian London, London's Tax Services | 12/17/2025

By Vivian London, London's Tax Services LLC | October 2025

If you're a small business owner, the next 90 days could either save you thousands in taxes or cost you just as much. The difference? Knowing what mistakes to avoid and taking action before December 31st.

After 20+ years helping businesses in Westchester and Rockland counties navigate their taxes, I've seen the same costly errors happen year after year. The frustrating part? Every single one is completely preventable.

The reality:

• 60% of Q4 tax-saving opportunities are missed entirely

• The average small business overpays by $10,000+ annually

• Poor Q4 planning creates devastating Q1 cash flow problems

Let's make sure you're not part of these statistics.

Update, September 2026: pair this with our new guide to small business tax deductions in 2026, including the year-end moves that must happen before December 31.

Mistake #1: Waiting Until December to Review Your Tax Situation

The Problem

Business owners get caught up in Q4 revenue goals, Black Friday planning, and holiday prep. Tax planning keeps getting pushed to the back burner. Then suddenly it's mid-December, and most powerful tax moves are off the table.

Real Cost Example

Sarah runs a consulting business that netted $180,000 in 2024. She finally contacted me on December 20th.

• Too late to set up a SEP-IRA for maximum deduction

• Too late to purchase qualifying equipment for bonus depreciation

• Too late to implement a retirement plan ($20,000+ deduction lost)

Result: Sarah paid $12,000 more in taxes than necessary.

The Fix: The October Tax Planning Meeting

Block out time in early October for comprehensive tax planning. Here's what to do:

1. Project Your Year-End Income Calculate expected Q4 revenue plus outstanding invoices. This gives you your estimated taxable income.

2. Review Your Deductions Pull your P&L through September 30th. Identify missing or low categories.

3. Calculate Your Tax Bill Estimate what you'll owe including federal income tax, self-employment tax (15.3%), and state taxes.

4. Identify Strategy Opportunities With 10-12 weeks left, you have time to make equipment purchases, increase retirement contributions, and time expense payments.

Action Step: Right now, block out 2-3 hours in the next two weeks for tax planning.

Mistake #2: Neglecting Required Estimated Tax Payments

The Problem

The IRS expects you to pay taxes throughout the year, not just on April 15th. If you're self-employed, you must make quarterly estimated tax payments. The fourth quarter payment is due January 15th.

Miss these, and you'll face penalties and interest, even if you pay your full tax bill by the filing deadline.

Real Example: Marcus's $83,300 Surprise

Marcus owns a digital marketing agency. In 2024, his business netted $220,000 profit. He didn't make any estimated payments, thinking "I'll deal with it in April."

April 15th arrived:

• Federal income tax: $48,000

• Self-employment tax: $31,000

• Underpayment penalty: $3,200

• Interest charges: $1,100

Total due: $83,300, all at once.

He had to take out a business loan at 12% interest, costing him another $10,000 over the year.

The Fix: Safe Harbor Strategy

Avoid penalties by using the IRS "safe harbor" rule:

Pay the lesser of:

• 90% of your current year tax, OR

• 100% of last year's total tax (110% if income was over $150,000)

Quick calculation:

• Look at last year's Form 1040, Line 24 (total tax)

• Multiply by 1.10 if income over $150K, else 1.00

• Divide by 4 for your quarterly amount

Pro Tip: If you've underpaid in earlier quarters, make a larger Q4 payment by January 15th to catch up.

Action Step: Calculate your safe harbor amount today. If you're behind, make a payment before January 15th.

Mistake #3: Missing Year-End Deductions

The Problem

December 31st is a hard deadline for many valuable deductions. Once January 1st arrives, these opportunities vanish, no extensions, no do-overs.

Top Missed Deductions

1. Section 179 Equipment Deduction Deduct up to $1,220,000 for equipment, machinery, vehicles, and software. Must be purchased AND placed in service by December 31st.

2. Bonus Depreciation (60% for 2024) Drops to 40% in 2025 and 20% in 2026. Timing your purchases matters.

3. Prepaid Expenses You can prepay and deduct insurance premiums, professional dues, software subscriptions, and rent (up to 12 months ahead).

4. Retirement Plans 401(k) plans must be established by December 31st. Employee deferrals must be deposited by year-end.

5. Bad Debt Write-Offs Review accounts receivable. Document uncollectible invoices and write them off.

6. Year-End Bonuses Bonuses paid by December 31st are deductible in the current year while rewarding your team.

Real Scenario: Jennifer's $8,750 Mistake

Jennifer runs a photography studio. In early January, she realized she owed substantial taxes and tried to:

• Purchase new camera equipment (too late)

• Set up a solo 401(k) (too late)

• Prepay six months of rent (too late)

Lost deductions: $35,000 = $8,750 in unnecessary taxes.

The Fix: Q4 Deduction Checklist

Equipment (By December 31st):

• Computers and software

• Office furniture

• Business vehicles

• Ensure delivery AND installation by year-end

Prepaid Expenses (By December 31st):

• Insurance premiums

• Professional dues

• Software subscriptions (annual payments)

Year-End Decisions:

• Employee bonuses

• Bad debt write-offs

• Retirement contributions

Action Step: Schedule a meeting with your accountant in October to review this checklist.

Mistake #4: Poor Record-Keeping and Documentation

The Problem

Without proper records, you'll lose deductions even if expenses were legitimate business costs. In an audit, no documentation = no deduction.

Common Documentation Failures

1. Missing Receipts IRS requires receipts for expenses over $75. Credit card statements alone aren't enough.

2. Inadequate Mileage Logs Need date, destination, business purpose, and miles, recorded contemporaneously, not reconstructed later.

3. Mixed Personal/Business Expenses Using the same account increases audit risk and makes bookkeeping nightmarish.

4. No Business Purpose Documentation Meals need who attended, what was discussed, and the business outcome.

5. Incomplete Asset Records Need purchase date, cost, description, business use percentage, and proof of payment.

Real Cost: Robert's $15,550 Audit

Robert owns a home renovation business. During an IRS audit, he couldn't document:

• $8,000 in materials (receipts lost)

• $12,000 in vehicle expenses (no mileage log)

• $6,000 in client meals (no business purpose)

• $15,000 in subcontractor payments (no 1099s)

The auditor disallowed $41,000 in deductions.

Total cost: $12,300 tax + $2,400 penalties + $850 interest = $15,550

The Fix: Simple Record-Keeping System

Step 1: Separate Accounts Get dedicated business checking and credit card. Never mix personal and business.

Step 2: Digitize Everything Photo every receipt with your phone. Store in cloud storage organized by month.

Step 3: Auto-Track Mileage Use MileIQ, Everlance, or Stride apps for automatic GPS tracking.

Step 4: Document Purpose Immediately Write business purpose on receipts when incurred. For meals, note attendees and topics.

Step 5: November Clean-Up

• Reconcile all statements

• Organize receipts by category

• Complete mileage log

• Gather 1099 forms

• Document major purchases

Pro Tip: Schedule a "bookkeeping power hour" every Friday in Q4. Spend 60 minutes catching up.

Action Step: Today, commit to one improvement, separate accounts or download a mileage app.

Mistake #5: Not Planning for Tax Payment Cash Flow

The Problem

A profitable year, but no money set aside for taxes. April 15th brings a massive bill you can't pay. This triggers IRS payment plans, maxed credit cards, delayed supplier payments, and constant stress.

Why This Happens

• You assume all money in your account is spendable

• You reinvest profits without maintaining tax reserves

• You underestimate your actual tax liability

• Q4 holiday expenses deplete cash reserves

Real Example: Lisa's $122,600 Crisis

Lisa's e-commerce business had $280,000 profit in 2024. She reinvested everything into inventory and marketing. No estimated payments made.

April 15th arrived:

• Federal tax: $62,000

• Self-employment tax: $39,600

• State tax: $16,800

• Penalty: $4,200

Total: $122,600. She only had $18,000.

She had to set up IRS payment plans, max out credit cards, delay paying suppliers, and cut employee hours. The stress nearly destroyed her business.

Understanding Your Real Tax Rate

Federal Income Tax: 10-37% Self-Employment Tax: 15.3% State Tax (NY): 6-9% Combined: 35-55%

For every $100 in profit, you might owe $35-$55 in taxes.

The Fix: Tax Savings System

Strategy 1: Separate Tax Account Open a "Tax Reserve" savings account. Every time money comes in, immediately transfer 35-45% to this account. Never touch it except for tax payments.

Strategy 2: Profit First Method Allocate every dollar:

• 30-40% → Tax Savings

• 40-50% → Operating Expenses

• 5-10% → Owner's Pay

• 5-10% → Profit Reserve

Strategy 3: Quarterly Tax Projections In October, calculate:

• Total year revenue (actual + projected)

• Minus all expenses

• Times 35-45% tax rate

• Minus payments already made

• Result = amount needed in savings

Strategy 4: December Cash Flow Review Before December 31st:

• Do I have enough for January 15th payment?

• Enough for April 15th?

• Can I cover Q1 expenses while keeping tax savings separate?

Emergency Option: If you realize you're short, contact your accountant immediately. Options include temporary loans, IRS payment plans (apply before April 15th), or strategically timing income/expenses.

Action Step: Today, open a separate tax savings account. Starting now, transfer 35-40% of all incoming revenue automatically.

Your Q4 Action Plan

Knowledge without action is worthless. Here's what to do in the next 30 days:

Week 1: Assessment

• Pull year-to-date P&L statement

• Calculate estimated tax liability

• Identify any payment shortfall

• Schedule meeting with accountant

Week 2-3: Implementation

• Open separate business accounts if needed

• Set up mileage tracking app

• Open tax reserve savings account

• Make Q4 estimated payment

• Finalize equipment purchases

Week 4: Final Review

• Confirm year-end purchases will be delivered by December 31st

• Verify documentation for major deductions

• Organize all records

• Complete November clean-up checklist

When to Get Professional Help

Consult a tax professional if:

• Business profit exceeded $100,000

• Considering major purchases

• Have employees or contractors

• Want to set up retirement plan

• Facing audit or owe back taxes

• Unsure if business structure is optimal

• Want peace of mind everything's correct

A good tax advisor saves you far more than their fee.

Ready to Take Control of Your Q4 Tax Strategy?

The next 90 days will determine whether you overpay by thousands or keep more of what you've earned.

At London's Tax Services, we've spent 20+ years helping businesses in Westchester and Rockland counties implement smart, legal tax strategies that save real money.

What We'll Do in Your Free Consultation:

✅ Review your current financials and tax situation ✅ Identify specific deductions you're missing ✅ Calculate your projected 2024 tax liability ✅ Create a customized Q4 action plan ✅ Answer all your tax questions in plain English

Book Your FREE 15-Minute Tax Consultation

Phone: 646-917-7714 Email: info@londonstaxservices.com Address: 75 South Broadway, 4th Flr, White Plains, NY 10601

Business Hours: Monday – Friday: 9:00 AM – 5:00 PM Sunday: 1:00 PM – 4:00 PM Extended hours during tax season

What Our Clients Say:

"Professional, articulate, always returns my calls with an answer." – Dahlia R., Bronx, NY

"Vivian is very knowledgeable and provided guidance which is what you need from your tax accountant." – Khalia S.

"As a small business owner this is exactly what I needed. Vivian goes above and beyond." – Tara G.

Don't Let These Mistakes Cost You Thousands

You've invested everything into building your business. Don't hand over thousands more than necessary to the IRS.

Take action today. Your future self will thank you.

Want this information as a quick reference? Download our FREE ebook: "5 Tax Mistakes Small Business Owners Make in Q4" with checklists and action steps.

About the Author: Vivian London is the founder of London's Tax Services LLC, serving small business owners in Westchester and Rockland counties for over 20 years.

Disclaimer: This blog post is for informational purposes only and should not be construed as tax advice. Always consult with a qualified tax professional before making financial decisions.