4a^3-16a^2b+16ab^2
= 4a(a^2-4ab+4b^2)
= 4a(a-2b)^2
(a+b)^2-4ab
=a^2+2ab+b^2-4ab
=a^2-2ab+b^2
=(a-b)^2
(x-y)^2-4(x-y-1)
= (x-y)^2-4(x-y) +4
= (x-y-2)^2