Here again we add and subtract suitable terms to bring it in a factorisation form. Notice that
x4+4y4=(x2)2+(2y2)2−2(x2)(2y2)−(2xy)2=(x2+2y2)2−(2xy)2
The second expression is in the form a2−b2, where a=(x2+2y2) and b=2xy
Hence we may write
x4+4y4=(x2+2y2+2xy)(x2+2y2−2xy)