4a^2-4(ab+4)+b^2
=4a^2-4ab-16+b^2
=(4a^2-4ab+b^2)-4^2
=(2a-b)^2-4^2
=(2a-b+4)(2a-b-4)
(x^2-2x)^2-2xy+x^2y
=(x^2-2x)^2+y(x^2-2x)
=(x^2-2x)(x^2-2x+y)
=x(x-2)(x^2-2x+y)
ax(y^3+b^3)+by(bx^2+a^2y)
=axy^3+axb^3+b^2x^2y+a^2by^2
=(axy^3+b^2x^2y)+(axb^3+a^2by^2)
=xy(ay^2+b^2x)+ab(xb^2+ay^2)
=(xy+ab)(ay^2+b^2x)