As you know, a 165 SR 15 is ideal. You could go with a 175, or a 185, but I don't think you will find tires in that size. About the biggest I would think of is a 185/65/15. You still won't gain the full benefit of that tire on a narrow wheel. Right now, I have that tire on the front of my car on a 6" wheel.
IMO, you shouldn't go any bigger on a 4
