JMO but it really depends on how good your bumper is attached to the frame and what type of bike rack you'll be using. I would buy the best bike rack you can afford. The kind that secures the bikes solidly and also a rack that doesn't wiggle. Too often you see bikes wiggling on the rack. That's the kind of movement that will create problems. Also you need to see how well you bumper is attached to the frame. You may need a couple gussets welded on for extra strength.
The problem with 4 bikes hanging off the rear is the cantilever affect. I think the best thing would be to have one of those fold down racks like you see on Forest River Wildcats. They're welded to the frame and support 250lbs.