First factor out 2
2(x^4 - y^4)
Rewrite it as ((x^2)^2) and ((y^2)^2)
Then you get..
2 ((x^2)^2) - (y^2)^2))
Now both terms are perfect squares, factor using the difference of squares formula, a^2 + b^2 = (a+b)(a-b)
a= x^2 and b= y^2
2((x^2 + y^2)(x+y)(x-y))
clean it up!
2(x^2 + y^2)(x+y)(x-y)
