We've had the older style Thule where you just having the bikes on a bar from the top tube. They swing around and bang into each other. I'm not sure but I think newer styles work better. Last year we got a
Saris Freedom rack that holds the wheels and down tube. The bikes don't bang into each other at all, they are very stable. I'm happy with that rack.
We have a 1" 1/4 hitch tube on our Subaru hatchback and I've been a bit concerned with its strength when we had our older heavy bikes on the rack. We've got carbon bikes now, half the weight from before, so I'm less concerned about bouncing weight on the hitch system. The weight of the rack its self is a potential issue too. Our old Thule is a big chunk of steel the Saris is steel too but not nearly as heavy.
If I needed to carry 4 bikes I would not want to hang them on the minimal hitch system we have. I think the tongue weight max is suppose to be 300 Lb. at most. 4 bouncing bikes on the back in a heavy rack could maybe be an issue. Other issues with the rear are the spray and grime from wet roads blasting the bikes and the fact you can't get into the rear hatch with the bikes on the rack.
If I was buying a new rack system today I'd go for the roof. The issue there is getting them up on the roof if you're shorter. If you need to remove the front wheels then you need space inside to keep them or another roof attachment for them. Roof systems also seem to be more pricy.