I've had a number of racks over the years Thule, Yakima, etc both roof and trailer hitch. I currently have the Yak trailer hitch model with the arms to mount the bikes. It will hold four bikes but it's a pain to get them on and off. Once on though they aren't going anywhere and I can affix any number of locks to make it nearly impossible to get off for anyone but the professional bike thief with a sufficient time on their hands.

If I had it all to do over again though I'd get the Yak trailer hitch model that the bikes stand on...the one without the arms; however, I don't think they are as secure though...at least casually looking at them they don't. I may be wrong on that.