Hey, finally a question I can answer authoritatively! I'm somewhat of an expert on the original Final Fantasy, and my website has quite a bit of space devoted to that game.
Four black belts is not difficult at ALL. In fact, I've beaten the entire game with a solo black belt without much difficulty. Four thiefs is also rather on the easy side, once you know what you're doing. (I'm assuming that this means going with four THIEFS and not class changing; four ninjas would be almost laughably easy.) Four black mages is a legit challenge; you have to know what you're doing and use your spell charges wisely. But with four different mages to spread around the castings, it can be readily done. Four white mages is definitely the toughest party of the six classes; you're severely limited on offense and defense, with nothing but healing magic to see you through. Four white wizards is much, much easier, but if you limit yourself just to the mages, you'll need to work your behind off to complete the game.
I outline how to do exactly that on my website, if you're curious or want some pointers. The only real challenge I haven't done yet is a solo white mage, which I'll have to get around to at some later date. 
End of 2008 totals: Wii 42m, 360 24m, PS3 18.5m (made Jan. 4, 2008)
