oh, silly me. I thought you meant you DID have a 4 star unit.

I misread your post.

well, here's the explanation :

the maximum a HUMAN player can get is 3 star. rebels (floods included) can get up to 4 stars (on tundra, for example, I think the marines protecting the megaturret are 4 star. and in campaign you see rebels and floods with 4 stars).

however, YOU are capped to 3 stars. you'll never have a 4 star unit, unless it's a spartan in a vehicle. you can kill a thousand units, you'll never get your 4th star.