Of course 4 bikes add trivial weight to the car itself but when several bikes are on a bike rack, there is a fair amount of force exerted by the rack where it rests against the car. My Rhodegear rack has foam around the rack to avoid scratching the paint but it still has to rest somewhere on the hatchback. On my Scion XB, one of the resting places is against the rear window and the other is below the window against the metal above the license plate. The lower resting point takes much of the weight. The farther out on the rack you have a bike, the more force it exerts on the lower resting place instead of supporting it by the upper straps. I know this from first hand experience. There are 6 straps in all to attach the rack - top, side, bottom but only the top straps support any of the weight.