Question 935134

How do I factor this?
(x^2+y^2)^2-4x^2y^2
Thank You. 
<pre>
{{{(x^2 + y^2)^2 - 4x^2y^2}}}
{{{((x^2 + y^2) - 2xy)((x^2 + y^2) + 2xy)}}} ----- Factoring as difference of two squares
{{{(x^2 + y^2 - 2xy)(x^2 + y^2 + 2xy)}}}
{{{(x^2 - 2xy + y^2)(x^2 + 2xy + y^2)}}}
{{{highlight_green(highlight_green((x - y)(x - y)(x + y)(x + y)))}}}