FFFg and FFg are the same. The FFFg is a finer granulation, and because we measure by volume instead of weight you get more powder with the finer FFFg. As a result you also get more bang.
I have always gone by this:
Fg is for cannons, FFg is for .50 and above, FFFg is for revolvers, and FFFFg is for the pan on a flintlock.