I just wondered if any other idiots have ever used it ... Thanks for the info guys....
I confess to having fired, unintentionally, a few rounds with 4F as main charge (25 gr) out of a 36 flint pistol. I forgot I had both a can of 4F and a can of 3F in my shooting box, and pulled out the 4F by mistake. I knew something was wrong or different because, surprisingly, the ignition didn't seem as fast and the recoil not as much as usual. The old Lyman Black Powder Handbook from the early 80s did list muzzle velocity and pressure data for revolvers with 4fg: 40 percent increase in pressure, maybe 15 or 20 percent increase in muzzle velocity when using 4Fg rather than 3Fg. In a flint gun, that extra pressure can get vented out the touchhole. In a percussion gun, I could see it resulting in more of a tendancy for hammer blowback and putting stress on internal lock components. Nowadays, as you may have noticed, conventional wisdom says reserve 4F for priming, no doubt unanimously from both gun and powder manufacturers.
Going back to your post that started this thread, what sort of fouling issues are you experiencing and with what load (of 3F)? SCL