Normally FFFg is used for 45 cal and under and FFg for 50 cal and up. However the 50 cal is one of those 'tweener' calibers. I find that 2 of my 50s shoot the FFFg better than the FFg. (about 100 gr by volume).
The difference is that the FFFg is finer and you will get more powder by weight for the same volume simply because there is less space between the granuals. This will result in increased velocities and pressures. There are charts available that list the velocities.