on a 4 pt, in an accident, when you go forward, you essentially pull the verticle belts up. this pulls the lap belt up and at the same time, can push you down in the seat. as the belts are being pulled up, you get pushed down underneath. i believe scroth put out something like that so you could see it.
as for the standard 3 pts, they are safer cause they can't submarine. when the belts tighten and you go forward into them em iin an accident, the cross belt tightens and therefore tightens the bottom. since the cross belt isn't directly connectec in the middle of the belt, it can't cause lift. this is why they are safe.
watch this vid. it KINDA explains it. notice they use regular 4pt harnesses as well to test, then the scroth. if you don't have an antisubmarine belt or antisubmarining technology, its not any safer than the stock 3 pt belts and in fact could be less safe.
Scroth is the only current belt that is approved for a 4 pt cause of its antisubmarining technology. however, 5 or greater pt harnesses anchor the submarine belt to the floor, so its impossible to submarine.
hope this helps. if you want piece of mind, you NEED either scroth belts or SFI/FIA homogated belts as they will hold you in a high speed accident such as rally or drag or track racing. its what all the big boys used.
before i bought my harnesses a while back, i researched this exact thing. those cheap 60 dollar harnesses you get from sprarco, omp, etc, are not safe. they are for show and are not DOT approved. the ONLY DOT approved belts are Scroth. even Sparco race belts and RCI, GForce, OMP (race versions) are not technically street legal, however, if you got pulled over for it and they are properly mounted, no cop in their right mind will pull you over for an SFI harness cause thats the safest belts you can possibly use.
if you go w/ these belts, they are prolly (and i can't say for sure) a step BACKWARDS in safety.