I used to agree with this. However, after running JRT's rig, and constantly tightening the lap belt, and talking with folks who've raced, the 5th is critical.Sub = submarine. As noted, its the crotch belt.
Frankly, I don't feel that (the 5th point in a 5 point..) it does much in a 4x4, if you have a relatively upright seating position.
5 points come from racing, where a reclined position puts the driver much more horizonal, and with the very high speeds involved, inertia coudl make a driver go out the bottom of the harness without that belt.
Myself, I'm putting 4 points with 2" straps into my upcoming flattie build.
Here's why: The lap holds you down. The shoulders are supposed to hold you back, but to do so they pull UP on the lap. This loosens the lap. With the 5th, this doesnt happen. I used to adjust JRT's harness every couple courses. I dont have to adjust the ftoy harness at all over a whole weekend. Same harness, bought from the same place. One is 5 point, one is 4.