Unfortunately, I don't think it's possible to do so because the "BAS_M433grenade" is not an object which you can CamCreate, and the "NearestObject" command only works with those. :-\
A work-around, though is this : remove the "BAS_M433grenade" using the "removeMagazine" command, and then add "GrenadeLauncher" with "addmagazine" : the M4 can use them...
The only thing is that you'll only carry a few grenades, whereas BAS units carry 5 BAS_M433grenades...
Hope this helps.