Java If-then-else RegEx

I need to create a regexp, that will do the following:

a,a,a,a,c - matches
c,a,a,a,a - matches
a,a,a,a,a - doesn't match

I will be using it in Java. In the place of 'a', can be 'b' - they are equal. Also, in the place there can be any other character. This is what i have came up with:

^(?=^(a|b)).*((a|b),){4}(?!(a|b))|(?!(a|b)).*(((a| b),*){4})$

It fails because it matches the 5 a's. I'm quite new to regexp, so I'm not aware of all the possibilities. It matches the 5 a's, because the first if fails, but the second does not. Maybe there is a simpler way to accomplish this? (Also why are the .* necessary in the middle?)