GT5 is the newest engine from Grubee. it has a slant head, speed carb with the pink airbox, 8mm head studs, slightly different shaped case, bigger motor mounts to fit newer frames (but not my old standard 1" tubes so i had to re-make mounts) and a few other things.
GT4's have a flat head, 6mm head studs, NT carb with black airbox.
both are 66cc's, and the 5 is supposed to have 2.6hp while the 4 claims 3hp (according to grubee website)
easiest way to tell the difference is the big ugly sticker on the clutch cover. GT5 is a skyhawk and the GT4 is a starfire. it should also say on the metal tag on the clutch rod housing, and failing that, check the side of the box it came in.
(edit) looks like Grubee's coming out with some more stuff, but as far as i know, GT5's are the only ones being imported right now.