I was looking at a IRS W-4 form recently and noticed that it had a section for "Other income (not from jobs)". So let's say you have a W-2 job and a 1099-NEC "job".
You're not employed, per se, with the 1099-NEC job, so could I put my expected earnings from my 1099-NEC job into the "Other income (not from jobs)" section of the W-4 in lieu of paying quarterly taxes via a 1040-ES?
The IRS doesn't care which pocket you take the money out to pay them.
If you have both a salaried (W2, as you say) job and a side contractor gig, you can most definitely adjust your salary withholding to also cover your additional tax liability from your side gig. Same goes with non-earned income (e.g.: you expect to have capital gains, or rental income, or win the lottery - you can adjust your salary withholding to cover for that).
From your question it was not immediately clear that you have this situation where you are both a salaried employee and a contractor, but from your comments you indicated that that is so.
Box 4a on the W-4 is labeled
Other income (not from jobs).
And the instructions for this line say:
Enter in this step the total of your other estimated income for the year, if any. You shouldn’t include income from any jobs or self-employment.
Contract work is a job, so I don't think you should include it there.
However, you can always use box 4c "Extra withholding" to increase your withholding so that you don't need to make estimated tax payments. The instructions for this line say:
Enter in this step any additional tax you want withheld from your pay each pay period, including any amounts from the Multiple Jobs Worksheet, line 4.
As long as the IRS receives the appropriate amount during the year, they don't care how you label it.
