This covers the simple analytical case. However, if this is the average return of your portfolio you may still end up using principal if the volatility of that return is anything but zero. In years in which the return is less than the average you will be withdrawing principal. So if that return is 3.5% one year then you have covered inflation and fees but your entire yearly spending of 4% is coming from principal. On the other side if you have returns of 11.5% one year then after you've covered your 7.5% (inflation + fees + yearly spending) you've added 4% to your portfolio.
Whether you actually do use up the principal over the long term life of the portfolio (your 30, 40, 50, etc year plan) depends on the sequence of those variations around the average return. If you get a lot of lower than average returns in the early years then you will start using up principal early in your plan. These are generally the withdrawal scenarios from the historical data that cause the portfolio to end the withdrawal period (30 years etc.) with no money left.
This indicates that it is a good idea for a FIREee to trade some average return for a lower volatility. How much average return should be traded off for how much volatility reduction? I'm not sure and I think that is an open question. It obviously, from ESRBob's explanation above, wouldn't be wise to go below 7.5% average return if you expect a 4% withdrawal. I would probably even suggest shooting for a little higher average return to balance against higher inflation (I think the average for the 20th century was 3.5%) and to provide a buffer.