-x^2y^2 + x^4 + 4y^2 - 4x^2
Separate into two parts to make simplification easier. Find the GCF of the values in each part.
-x^2 (y^2 - x^2) + 4 (y^2 - x^2)
Combine the factors and because the two in parenthesis are equal, eliminate one.
(-x^2 + 4) (y^2 - x^2)
Hope this helps!