c4 cycle in photosynthesis occurs in monocots or dicots
monocots don't lack bundle sheath cells.  In fact, they are much more likely use c4 respiration than dicots.  There are actually about 10 times more c4 monocot species than c4 dicot species.  the c4 cycle is really primarily a monocot adaptation.  I think that the confusion often comes from the fact that dicots appear to be "more sophisticated" than monocots when you look at them, with their pretty, well organized vascular bundles and branching veins, which makes it intuitive to assume that they would be the ones using the "higher tech" c4 system.