(1-a²)(1-b²)-4ab
=1-a²-b²+a²b²-4ab
=(1-2ab+a²b²)-(a²+b²+2ab)
=(1-ab)²-(a+b)²
=[(1-ab)+(a+b)][(1-ab)-(a+b)]
=(1+a+b-ab)(1-a-b-ab)