Generally 4f is for priming flintlocks, small and faster burning.
Manufactures recommend 3f for guns 45 cal. and smaller. I always used 3 f in my 50 cal. When I was shooting a lot at matches, that was the comman useage.
Larger than 50 cal, used 2f. 1 f was for cannons.
Small granules burn faster and more pressure. I could probably use 2f in my 54 cal, but would reduce from 110 grains to around 80 gr.
Use the manufactures recommendations for each gun. I tested all my guns when built, with a stout charge, and fired with a string when they were set off the first time, while I stood behind something like tree.