The original trinity study that arrived at the 4% rule was based on withdrawing cash once per month.
In practice what that would mean is if you have bond interest or dividends greater than what you're supposed to withdraw for the month they'd get reinvested, and if you don't have enough you'd sell stocks/bonds to make up the difference.
In months when you'd need to sell something, you'd choose whichever you have too much of in your portfolio. For example, if your target asset allocation was 90% stocks/10% bonds, but stocks were having a bad month and bonds a good one, your actual ratio might be 88% stocks and 12% bonds, so you'd only sell bonds that month.