Buds sells MSAR E4s, they finally fixed their webpage to show them. All the models that start with 250xxx are E4s. Buying a regular STG from them is a roll of the dice as to what gen you'll get. Their E4s are also older stock with the 5 XM30 mags that don't work. The good news is they have optics models and the barrels are threaded 1/2x28, the bad is they are expensive and slow as hell to ship.
STG gen 1 & 2 have FA, gen 3 doesn't and gen 4 gets the square lock that's directly compatable with the E4 stock. Of course the E4 takes AR pattern mags. You pretty much have to send in a gen 1-3 to MSAR to be converted and then your STG stock doesn't work anymore.
If you're thinking of getting a STG cheap and adding an E4 stock for a cheap E4, it ain't going to work. The E4 conversion is ~$450. It's better to just buy a E4 from Ratworx. If you want both types, again Ratworx is best because their STGs are 4th gen and they can get you a E4 stock too. The E4 will probably also have the new steel mag catch, come with P-mags but will be 13x1 LH threaded and no optic.