And Yes, it will work way better. The purpose of the submarine belt is to keep the lap belt held down. A lot of people make the mistake of tightening the shoulder belts to keep them DOWN in their seat, that's how broken backs happen. Your lap belt should keep you down, and your shoulder belts should keep you back.
The problem with a 4 point is when you tighten the shoulder belts, it lifts your lap belt, thus introducing the need for a 5th strap. For your lower speed rock crawling stuff, you'd be better off keeping your lap belt tight, and your shoulder belts tightish. That way in a roll over, your butt will stay planted.