If you reread Thunder's post, he said TM would be your best bet if your looking for a quality plastic aeg.
If you meant plastic as in the Canadian approved airsoft guns with the clear plastic lower receiver, then G&G, as that is the only available M4A1 "cansoft". If you are of age, however, you can take the time to get age verified and get your hands on a quality aeg in the classifieds
