How 'bout any car you want for daily use and a trailer? I pull one of these with my MINI (Class 2 hitch, don't even need a big hitch for it)
http://www.hdtrailer.fix.net/
Yakima just bought this equivalent trailer last year
http://www.yakima.com/productcategory.aspx?id=52

On the sportsrig we can get, 2 velomobiles or 3 trikes or a combo of trikes/bikes. I would guess 4 bikes wouldn't be a problem.
The gas mileage difference on my friends car is ~28 mpg to ~27 mpg on his highway trips. I've only used mine short distances so can't tell you any difference in the MINI.