The greatest common factor is the greatest number that can divide the terms. The greatest number that can divide each term is 4, and the greatest power of x that can divide each term is x^2. This is because it can't be any number greater than 4 since that number would never be able to divide into 4. This is also the reason why x^2 is the greatest power possible. It cannot be greater than anything divisible by itself. Thus, you multiply 4 by x^2 and the answer is 4x^2.
