Rob got it pretty good, there isn’t really much different about them. They might market some chassis as 4-cycle but the biggest thing is just lower quality components because the 4-stroke stuff is more of a budget class. There’s some tubing differences, although how much of a change there is has not been determined, and they’ll typically run aluminum instead of magnesium parts and try to keep costs down.
Try to find a good deal on a kart and worry about the chassis type later. A generally agreed upon number is that a better chassis will get you about a half-second or so of laptime