Verified answer

z^4-y^4

(z^2-y^2)(z^2+y^2)

(z-y)(z+y)(z^2+y^2) --------------final answer

(z^2 - y^2)(z^2 + y^2)

and that factorises some more:

(z - y)(z+y)(z^2 + y^2)

nvm i messed up