As I've done myself in a pinch occasionally.
If you have 3-FFF available. And needing 4-FFFF. Just take a coffee cup saucer or small mirror glass. Pour a little 3-FF on to it and proceed to chop it up into a smaller grain size with a stiff edge safety razor blade. 2-3-& 4-ffff are the same powder in a given powder brand. They differ only by their kernel-or-grain size. ( F-grade ratings.)< That's all that separate them from each other.___ (F) by the way stands for French thanks to Mr. DuPont giving meaning to the letter F shortly after 1802.