Another reason I've never been too slavish about using the "4% rule." Any way that recognizes the taxes (either discounting your taxable stash or calling the tax taken from every withdrawal an "expense") will get you where you need to be. It can get complicated and there are tricks to play to lower the taxes (take some from qualified sources - 401(k) tIRA, etc. and some from, say cash in your cash bucket, already taxed money such as outside mutual funds, tax-loss harvest, etc. etc.) Point is you probably "should" (maybe too strong a word) recognize that most tIRA and 401(k), etc. balances are of less value (for most of us) than cash under the mattress, already taxed money, etc. It's all in the accounting and the intricate games the convoluted tax system forces us to play (if we wish to spend more on geisha girls, booze, etc. - or just wasting it, heh, heh.) How you account is up to you, but the 4% rule can be applied any way you want as long as you do account for the taxes. Sorry if this isn't the easy answer OP was looking for.
By the way, this is one reason I've become an advocate (too late for me) for not overloading on qualified money during your w*rking years. Emphasize Roths and other tax-already-paid vehicles unless you're situation is favorable to shielding now to pay later. Eventually RMDs will rear their ugly rear and leave far fewer tax options - so plan ahead. In any case, it's impossible to play it perfectly. After all, they can change the rules after you've already started playing, so YMMV.
YMMV