If your RSUs vested earlier this year and your company withheld at the 22% supplemental rate, you may already know the withholding didn't cover the full tax cost. The vest is behind you. The gap is known. What remains is a Q4 decision about how to close it — and the clock runs to January 15, 2027.
The three options — increasing W-4 withholding before year-end, making a Q4 estimated-tax payment by January 15, or doing nothing and accepting a §6654 penalty at filing — all work. They work differently. The right one depends on income timing, liquidity, and where your year-end tax position actually lands relative to the safe-harbor floor. This piece walks the decision sequence.
Why the supplemental rate undershoots for hybrid earners
The supplemental withholding rate — the flat rate applied to RSU income at vest — is set under Treas. Reg. §31.3402(g)-1, the regulation that implements the supplemental-wage rules under IRC §3402. For 2026, the rate is 22% on aggregate supplemental wages up to $1 million; it jumps to 37% mandatory above that threshold.
The mechanism is designed for administrative simplicity. Your employer applies a fixed rate to the supplemental wage item — the RSU vest — without knowing what else is on your return. It does not see your S-corp K-1 pass-through income. It does not see income from other sources. It calculates withholding against an incomplete picture.
For a hybrid earner whose effective marginal rate is 32–37% when all income streams combine on the same return, 22% is a structural undershoot — not an error in execution but an error built into the mechanism. The RSU Vesting Tax Trap covers the vest-time mechanics; this piece picks up at Q4 — the gap is established, and the question is what to do about it now.
The three-option decision
A hybrid earner in this position has three paths into year-end. Each carries a distinct mechanism, a distinct trade-off, and a distinct use case.
| Option | Mechanism | Key trade-off | Works best when… |
|---|---|---|---|
| A — Increase W-4 withholding | Submit a revised Form W-4 (Line 4(c), additional withholding per pay period). Under §6654(g)(1), total wage withholding for the year is treated as paid in four equal installments across the four installment due dates — regardless of when the withholding actually occurred during the year — unless the taxpayer elects to use actual withholding dates. | Under the default Form 2210 method (four equal 25% installments), §6654(g)(1) ratable treatment reconstructs installment credits after the fact — a W-4 increase large enough to make total annual withholding meet the required annual payment retroactively clears the installment picture. If the increase is not large enough to fully cover the required annual payment, penalty accrued on the shortfall from earlier installment due dates remains, calculated at filing on Form 2210. Under Schedule AI (annualized income installment), a Q2-front-loaded vest may produce a Q2 required installment larger than 25% of the annual figure — total Q4 withholding may then clear the annual safe-harbor but leave a Schedule AI Q2 shortfall uncured. | Remaining Q4 pay periods can absorb the gap; reader prefers an automatic mechanism over a manual payment; safe-harbor is assessed on annual-total basis (default Form 2210 method). |
| B — Q4 estimated-tax payment | Pay via IRS Direct Pay or EFTPS; Q4 installment deadline is January 15, 2027. | Does not retroactively cure Q1–Q3 installment-method shortfalls. Covers the Q4 income period going forward. Requires liquidity for a lump payment before mid-January. | The gap is driven primarily by Q4-period income (Q4 RSU vest, year-end K-1 distributions); reader on annual safe-harbor method; has liquidity available for a January payment. |
| C — Do nothing / accept §6654 penalty | No action before January 15. The §6654 penalty is calculated at filing via Form 2210. | The §6654 rate is tied to the federal short-term rate plus 3 percentage points under §6621; for Q3 2026 the applicable rate is 7% (per Rev. Rul. 2026-10, IRB 2026-22). Reader retains full liquidity until the April filing deadline. | Safe-harbor already cleared through Q1–Q3 payments; the shortfall is modest; the after-tax cost of the 7% annualized penalty is less than the liquidity cost of a Q4 payment. |
The anchor for all three options is the §6654 safe harbor. Under §6654(d)(1)(B) and (C), a hybrid earner whose prior-year AGI exceeded $150,000 must pay in 110% of the prior-year tax liability to avoid underpayment penalties. (Below that AGI, the floor is 100% of prior-year tax; the 90%-of-current-year alternative also applies but is harder to rely on mid-year.) The Q4 decision is fundamentally about the distance between total-year payments and that floor.
On the Schedule AI nuance in Option A: The default Form 2210 approach divides the required annual payment into four equal 25% installments. Under Schedule AI, the IRS annualizes income through each quarter and recalculates the required installment based on actual income received — which can make Q2 particularly large for a hybrid earner who vested a large RSU block in May or June. If Schedule AI applies to your situation, a Q4 W-4 increase resolves the prospective shortfall but does not retroactively satisfy the Q2 installment requirement. For a deeper treatment of Schedule AI mechanics, see Werner's Q3 estimated-tax piece.
The hybrid-earner complication: S-corp K-1 stacks the marginal rate
The employer's supplemental withholding calculation was performed against the employer's view of the world: W-2 wages and RSU income. It had no visibility into the S-corp K-1 pass-through.
K-1 pass-through income is not subject to withholding. It flows directly onto the return at whatever rate applies to the top of combined income. For a hybrid earner with a day-job W-2, an RSU vest, and an S-corp K-1, the marginal rate on the highest dollars is typically determined by all three streams stacked — not by the RSU income in isolation. The employer calculated 22% against a number it could see; the actual rate applies to a number that includes income the employer never had access to.
One entity-structure point worth noting for hybrid earners who draw a salary from both an employer and an S-corp: the $1 million aggregate supplemental wage threshold under Treas. Reg. §31.3402(g)-1 is applied per employer. W-2 wages from the S-corp and W-2 wages from the day-job employer do not aggregate for purposes of the 37% mandatory-withholding trigger. Each employer calculates independently. See IRS Publication 15 for the per-employer calculation mechanics.
The effect of S-corp K-1 stacking on the hybrid earner's total marginal rate is covered in the S-corp entity-choice framework at When NOT to Elect S-Corp: Five Scenarios.
How to size the gap and decide
The decision sequence is arithmetic once the inputs are assembled:
- Determine the safe-harbor floor. Pull the total tax line from last year's Form 1040 (line 24 on the 2025 return). If prior-year AGI exceeded $150,000 — or $75,000 for married filing separately — the floor is 110% of that figure. This is the number that needs to be paid in by January 15 — via withholding, estimated payments, or a combination — to avoid underpayment penalties entirely.
- Sum Q1–Q3 withholding and estimated payments already made. Federal income tax withheld appears on paystubs and will reconcile to Box 2 on the W-2. Quarterly estimated payments are confirmed via IRS account transcripts or the EFTPS payment history.
- Calculate the remaining gap to the safe-harbor floor. Subtract Q1–Q3 total payments from the floor. The difference is the amount that must land by January 15 to clear the safe-harbor on an annual basis under the default Form 2210 method.
- Determine how much Q4 W-4 withholding can absorb. Remaining pay periods multiplied by available additional-withholding room per period gives the W-4 capacity. Anything beyond that capacity requires a direct estimated payment.
- If the safe-harbor is already cleared, Option C is arithmetic. The §6654 penalty rate is modest — if the remaining gap is small and the safe-harbor floor is already within reach, the penalty math may favor holding liquidity until April. IRS Publication 505 provides the underpayment rate worksheet for this comparison.
The Q4 decision does not require predicting the full tax year with precision. The safe-harbor floor is a prior-year figure — fixed, lookupable, and independent of how the current year ultimately lands. The reader who clears 110% of prior-year tax through January 15 has navigated the penalty exposure regardless of where the current-year liability settles at filing.