You absolutely can go with a 4 man rotation if you want to, though I wouldn't suggest it, because your pitchers won't recover all of their stamina. Only time I have actually went with a 4 man rotation is in the playoffs.
I just tested this, to make sure it works, and it does in fact work. It is best used when an off day falls on the day your 5th starter is scheduled to pitch, that way the other 4 starters will pitch on their normal rest, even though the 5th starter's spot is skipped.