To achieve the 4% SWR, I expect to have to withdraw principal. The bulk of my investments for the foreseeable future will be invested in stock indexes (with 20-25% in other assets such as bonds), generally generating higher levels of returns than those found in munis. My own modelling uses a 6% annual rate of return (as I'd rather be conservative in my estimates).
However, please note that I am several years away from FIRE. Those few families that I know that are retired are only spending from their earnings (dividends & interest income) and not touching their principal. They, however, have other forms of income such as SS. Ideally, I think I'd prefer to only spend earnings (dividends & interest income) and leave principal.
Separately, I anticipate purchasing Munis when I am closer to the capital preservation phase of my financial life --> right now it is accumulation and growth. Just to note, its fairly easy to buy Munis, you can purchase in individual bonds in $10K increments, so spend $100K on ten different Munis (I've watched how the retires I know do this). This however will largely be an income tax play rather than a step to FIRE.