I did this last week... I too have an inappropriate 4x4 with a spare wheel on the back (defender in my case).
I went to my local trailer center and they had spaces and bolts. I used 3 1.5 inch spacers and a tow hitch that has a longer neck. I used a pendle carrier to hall 4 bike to glentress from Chester this weekend.
Points to note. I still had to take a pedal off the bike closes to the wheel. Also the chap at the center said it is illegal to tow with more than 1 spacer and it was possible that I could get pulled over for having an illiagal tow bar, but he said if I kept (which I do) the pendle bit always attached I should be okay as it's obviously used as a bike carrier.
I didn't bend my bolts with 4 160mm travel bikes,but they did lossen off, bearing in mind I had washers and 2 of us gave them a pretty hard swing to tighten. I just put locking nuts on an it was fine. Just saying keep an eye on them at first.