The 'bible' of what cars can be towed 4 down is the Motorhome magazine annual list.
Downloadable Dinghy Guides | MotorHome Magazine
Towing 4 down is easier than dealing with a tow dolly, but more expensive and limits you to the one modified vehicle.
Any car you're thinking about on the above list, verify by looking up the Owner's manual and reading it in the manual. Don't go by salesperson's or other 'expert's' word.
Also make sure a base plate is available for a model you're about to buy. Use Google to find the various manufacturers of towing equipment. Buying tow bars or brake systems on Craig's list, Ebay, etc. can also get you in the game at reduced expenditure. I got a Blue Ox tow bar from Craig's List for $150 and a Roadmaster brake system for $300 from Ebay, so bargains can be found.