The biggest issue with four down and a dolly or trailer is what to do with the dolly or trailer when not using the coach or at a campsite. I started with a dolly because I had a car that I was attached to and couldn't tow it four down. After several trips of dealing with unhooking and hooking up, storing the dolly and having to move it around to store, including one time of losing control and ending up with a very large gash in my leg, we went to four down.
Added cost was +/-$2500 for tow bar, base plate, cables, auxiliary brake, etc not including cost of purchasing a new toad. Actually like my Saturn Vue better as my second car and toad and would never go back to towing a trailer or dolly.