Mountain bike or motorcycle?
In general, for more than two bicycles, I've had the best luck with vertical bike racks and currently use one from
VelociRAX. I have their wide 5-bike rack, and aside from it being relatively heavy and cumbersome to get into and out of the receiver, it works quite well and I've been impressed by it's quality and construction. I mostly use my F250 when I go riding, but plan on building a bumper to accommodate it on my EB as well. I use velcro straps to lock the front brakes in the rack in addition to the normal rubber straps, as it tends to keep everything where it belongs, even on rougher terrain, high speed hits, etc.
If you're talking motorcycles, then a simple 4-bike trailer would be the lightest and easiest thing to tow, but offers almost no safety against theft, whereas a small enclosed trailer would be more versatile, safer and more secure, but would be more trailer to pull.