You will enter the tax withheld as reported in lines 4 and/or 16 on your Form 1099-B when you enter that form as part of an investment sale entry in TurboTax.
You enter investment sales in the Wages and Income section of TurboTax, then Investments Sales, then Stocks, cryptocurrency, Mutual Funds, Bonds, etc... Upload your Form 1099-B or choose Stock, Bonds, Mutual Funds as the type of investment you want to enter. You'll come to a screen where you can enter in your sales proceeds, cost basis and taxes withheld.
There is no option to enter box 4 on Turbtax 1099B. I have boxes a, b and c only with no other options.
The federal withholding as indicated by @ThomasM125 is entered into the 1099-B. You must select 'I have more info to enter that I don't see'. See the image below.