Stuck on my Game of Life

here is my attempt to put all three tests together into one tower.
http://twitpic.com/5ttqf0
Something is wrong with my logic on the first “floor” For the case where it is testing for
0001
0010
0100
1000
I used three xor gates where AB goes into XOR(1) CD goes into XOR(2) and XOR(1) XOR(2) goes into XOR(3)
but that fails for the cases such as A=1 B=1 and C=1
still lots O fun though.