I have no problem finding 4F so thats what i've been using. Very fast ignition times. I tried 3F and it works as well but I did notice a very slight delay in ignition vs 4F. There is ways to crush 3F down to 4F although i too am a beginner with flintlocks and do not know the exact method. VERY helpful guys over at the muzzleloader forum , they probably can help :
http://www.muzzleloadingforum.com/fusionbb/index.php?