Number of optically active isomers of [Co(NH3)2Cl2(NO)(OH)]?
Try drawing out [M(ab)(ab)(cd)]: you'll see that it's actually optically inactive!



So, there are 2 optically active isomers, which along with their respective enantiomers adds up to 4 (M[(ab)(ac)(bd)] , M[(ab)(bc)(ad)],  as you've mentioned).