Factoring x^2 - y^2 = (x+y)*(x-y) is the standard shortcut, known to all school students of adequate age/grade, who learn/study Algebra. So apply this shortcut twice to get x^4 - y^4 = (x^2)^2 - (y^2)^2 = (x^2 + y^2)*(x^2 - y^2) = factor (x^2-y^2) again = (x^2 + y^2)*(x+y)*(x-y).