I don't really understand how it can possibly take 4 STR = 1 Attack Power. (Or do you mean 4 STR = 1 Damage?)

If you look at materia, STR materia IV is ~16 str. While Heaven's Fist materia is ~26-30 Attack Power.

If STR materia is only 1/4 of a single attack power as far as damage output goes, that would make the attack power materia a whopping 8 times more effective than the strength materia. I.E. virtually securing it as the ONLY materia suitable for damage dealing. Something doesn't seem right with it.