If you have a waged W-2 job and you want that job to withhold taxes for the other income you have ( the Sch C income from your side hustle) then you DO enter the net income expected from that job.
OR you can not put that income on the W-4 and either enter an amount per paycheck of your choosing on line 4c OR make quarterly estimated payments OR simply owe on your tax return and pay the taxes then.
All 4 methods are acceptable ... it is your choice.
