a & b = no errors
c & d = parity errors

a & c = no errors
b & d = parity errors

a & b = no errors (repeated test, but it never hurts to be sure)
a & c = no errors (again, repeated test)
b & c = no errors (we expect this, because we suspect d is the bad memory)

d & a = parity errors (probably because of d)
d & b = parity errors (again, probably because of d)
d & c = parity errors (really pretty sure now, d is junk)