i would not want 4 bikes on a rear mounted carrier, 2 was bad enough. Cant open the boot without removing the rack and the current cars that have minimal shut line gaps or glass to the edge of the tailgate mean they are not viable for many cars
Thule, pendle or Saris are the go to brands
I have a roof mount for the mountainbike and as long as its tightened correctly doesnt move. Ive abused the speed recommendations with no issues. 3 downsides, being tall enough/strong enough to lift the bike over head height onto the mount, any mud caked on flies off as you drive leaving the roof spattered and any anyone behind, and height restrictions- many bikes have been dynamically manipulated having connected with said restriction.
My road bike is light and clean enough to fit in the back of the car.