When you expand the binomial, you isolate each of the parts. So, (x-y)^4 would turn into...
x^4 - y^4
The powers stay the same because (outside the parenthesis) you multiply them with whatever is inside the parenthesis. In this case, would be 1. All variables and numbers without a listed power have a power of one. Multiply 4*1 to get 4.
