Plenty of folks are towing them 4 down. Blue Ox has a base plate. There is a company in the UK that sells a tow bar and supplemental vacuum brake system.
I don't exactly know the hows/whys ... but the auto can be towed 4 down, because it is not a "true" auto. It is a manual without a clutch, and no torque converter ... ??? Some design that I am still trying to figure out the details.
The Smart is just over 1800lbs.

I towed a Suzuki Samurai with our other Class B on a 2004 E350. I used a United Gear supplemental braking system even though the Sami weighed about 2000lbs. I don't like "marginal" braking scenarios.
I'd get another Suzuki if I could find the right one. Although riding around in a 1980's Samurai is only slightly better than a golf cart ! The Smart would have a/c, easily maintain highway speeds and modern safety restraints. An MR2 would doo all of that also, but would it have a place behind the seats for my dog ? She goes everywhere with us ! Which is another reason I am seriously considering a Smart car as a dinghy.