I just had this happen to me on Friday, and the 4th screw was stripped before I even tried to loosen it. I will say noise/vibration seemed to be fine with 3, but I'm OCD with stuff like this. I went in to the Apple store the next morning, but they didn't have any in stock and wound up pulling one out of their machines in the back for my replacement. 😀
PS If you don't feel like going in, the guy who helped me used what looked like a needle nose pliers to get it out.