B1 B A1 A->Y
0 0 0 0 Y=1 
1 0 0 0 Y=0 
1 1 0 0 Y=0 
0 1 0 0 Y=0 
0 1 1 0 Y=0 
1 1 1 0 Y=0 
1 0 1 0 Y=0 
0 0 1 0 Y=1 
0 0 1 1 Y=0 
1 0 1 1 Y=0 
1 1 1 1 Y=0 
0 1 1 1 Y=0 
0 1 0 1 Y=0 
1 1 0 1 Y=0 
1 0 0 1 Y=0 
0 0 0 1 Y=1 
function: Y = (!B1 && !B && !A1 && !A || !B1 && !B && A1 && !A || !B1 && !B && !A1 && A)