（1-x²）（1-y²）-4xy
=1-x²-y²+x²y²-4xy
=1-2xy+x²y²-x²-y²-2xy
=（xy-1）²-（x+y）²
=（xy-1+x+y）（xy-1-x-y）．
故答案为：（xy-1+x+y）（xy-1-x-y）．