(x-y+4xy/x-y)(x+y-4xy/x+y)
=[(x-y)^2+4xy][(x+y)^2-4xy]/[(x-y)(x+y)]
=[(x+y)^2(x-y)^2]/[(x-y)(x+y)]
=(x-y)(x+y)
=x^2-y^2