It’s perhaps just a little different depending on how you choose to withdraw funds using a % of remaining portfolio method.
Say you withdrew your 4%, then found that you didn’t spend $10K of it, so the next year you decide to withdraw $10k less. Now your withdrawal rate is slightly lower. No prob.
What about that extra $10K you left in your portfolio? Well it increased your remaining portfolio a bit, so if you stuck to your 4% of remaining portfolio, your future income increased $400 a year assuming little volatility.
Or you could choose not to be that strict and just perhaps keep track of some unspent funds that were left invested but could be withdrawn as extra later if needed. This would generally be fine unless you went through a period where the portfolio took a bad hit in which case your annual income will go down plus that extra you planned to perhaps take in the future would also be hit. Temporarily.
Those are just the trade offs between keeping unspent funds in potentially volatile long-term investments.