In the "old days" I'm sure they only carried one kind of powder and loaded and primed with it. I think 4F will give you a bit faster ignition, however I usually hunt with 3F loaded and primed.
A delay or "hang" fire would/ could happen if your pan were dirty or the touch hole clogged and could happen with either 4F or 3F. These guns, (flintlocks) go off not from a "fused" effect between pan and main charge but rather from the heat of the burning powder in the pan going through the touch hole and igniting the main charge. Many people do not understand this and tend to over charge the pan which leads to fouling in the pan and touch hole. If the gun is built correctly, and the touch hole is centered at the top of the pan, it takes very little powder to set it off.
One thing, if you shoot at a match and prime with 3F make sure anyone standing to your right is well out of the way, otherwise they will get a "shower"!