ab-(a-2)(b-2)=1/2·ab
∴(a-2)(b-2)=1/2·ab
ab-2a-2b+4=1/2·ab
1/2·ab-2a-2b=-4
∴ab-4a-4b=-8
∴(a-4)(b-4)=ab-4a-4b+16=-8+16=8