You do not need to do quarterly estimated payments, at all.
This is because "withholding" (but not estimated quarterly payments) is considered as withheld equally throughout the year. So, your lump sum withholding, in December, is probably the only tax payment you need to make during the year. If 20% doesn't come to $6000*, you can ask your 401k administrator to withhold the full $6000.
*This assumes $6000 was your tax liability (line 24 of form 1040), not your balance due (line 37).
[EDIT] If your liability is more, you can still have the 401k withhold the full amount (your 2024 line 24).
