One place that lists the "4 down" towables is: http://www.motorhomemagazine.com/
Scroll down the main page, about half way down on the right side you will find links to their dinghy guide from 2002 to 2009. Note that their listing is according to what the manufacturers state. Also, if you are a FMCA member, I believe they have a similar listing on their site.
If your vehicle is not towable "4 down", try this site:
Remco has alternative solutions to towing 4 down including lube pumps, drive shaft disconnects, etc.