\(x^4+4y^4\)
\(=x^4+4x^2y^2+4y^4-4x^2y^2\)
\(=\left(x^4+4x^2y^2+4y^4\right)-4x^2y^2\)
\(=\left[\left(x^2\right)^2+2.x^2.2y^2+\left(2y^2\right)^2\right]-\left(2xy\right)^2\)
\(=\left(x^2+2y^2\right)^2-\left(2xy\right)^2\)
\(=\left(x^2+2y^2-2xy\right)\left(x^2+2y^2+2xy\right)\)
x4+4y4+4x2y2-4x2y2
=(x4+4x2y2+4y4)-4x2y2
=(x2+2y2)2-4x2y2
=(x2+2y2-2xy)(x2+2y2+2xy)
