The situation is..., the finer the granules, the faster the detonation, so the higher the pressure spike.
That being said, I use 3Fg in my .54 flinter for BOTH the pan and the main charge. I use 70 grains for the main charge, and I used the same load in my old .50 flinter from Cabela's. So with a moderate load you should have no worries.
As for the 4Fg, it is very fine, so it will tend to absorb moisture. As such, try it in your pan, and try the 3Fg as well, and if you don't see a huge advantage then use only the 3Fg for both. Remember it will be one thing for you to shoot it on the range, and quite another to walk around the woods for a while, hunting. In the second situation, the 4Fg priming powder in the pan will be absorbing moisture, and will slow down the detonation or perhaps prevent it. When in the field I dump the pan powder once an hour and reprime, even though it's 3Fg. Haven't had a misfire in the field yet (knock on wood).
I use one horn for both prime and main charge, as it's so much simpler, and my fixed measure throws 70 grains, and that's the charge I also use for target shooting, SO..., I know where my rifle shoots each and every time from 25 yards to 100 yards (I don't like the idea of half charges for target shooting or small game).
BTW a pound (16 oz.) of powder is 7000 grains, which equals 100 shots if each is a 70 grain load, minus that lost when used to prime the pan. Just fyi.
LD