Car hitches come in 1 1/4" hitch and 2" hitch sizes. The former is dubbed (in North America, at least) either class-1 or class-2, and the latter is dubbed class-3. Class 4, 5, and 6 also exist, but do not seem relevant for cycling.
What are the rules of thumb regarding the carrying capacity of 1 1/4" class-1 vs 2" class-3 hitches? In particular, if a car's chassis seems like it will not handle a 2" hitch (because neither the dealer nor the major independent installer offers it), does mounting a 4-bike rack—loaded with four MTBs—remain an option?
I am reading that the carrying capacity for a 1 1/4" class-1 hitch is 200 lb, which would allow for four MTBs weighing 120 lb and a 4-bike rack weighing 80 lb. Is this not recommended because it provides no margin of safety?
I am aware that the best way to carry four (and more) bicycles is to have a truck, but aside from that option, what are some of the reliable ways (no sway; stable at highway speeds) for carrying four bicycles on a hatchback? Is there, for example, an option to use a 1 1/4" hitch, but to reduce the torque on the hitch by strapping a hitch-mounted bike rack to roof rails?