Depends whether you mean in the draft, or to run with all season, plus your pitching strategy.
Obviously you want to use all 1250 innings, so with 4 starters, even with 4 stud pitchers who stay healthy and log 220 innings each, you would probaly find yourself coming up short. (880 innings leaves 370, meaning you'd need at least 5 and maybe 6 releivers to meet your quota)
If you're going for a middle releiver strategy (which i do) then you could just about make it with 4 starters, but you'd probally be best off with 5 so you can be a bit more selective over matchups, and have cover for the inevitable missed time.
If you're being more orthodox, you'll need at least 5 starters, and 6 wouldn't hurt, but 4's not a bad number to draft, just as long as you aim to pick some up in season through trades or pick ups.