In short, 5 and 6 point harnesses have the additional point between the legs. think reclined/relaxed seat angle for endurance. 120 to 0mph would be a similar analogy to you being toothpaste in a tube, you will squish/slide out the one area that is not restrained... You would be lucky if you only took an arrow to the knee...
So if you looking for a 4point, might as well go 5/6.
Submarining under the belt. Having the belt buckle ride up too high crushing your soft fleshy gut in an impact. I think the only exceptions are the Scroth ASM 4 points but I don't know why anyone would not purchase a 5 or 6pt. It just makes sense.
Also "tuner" style belts (non camlock) are scary too.