So, here's one to consider. Over on Vans Air Force forums, on the front page is a survey on whether Van's should build a back country super cub type version RV.
As stated before, the Return On Investment for Kitfox to design a 4 place, IMHO, would not be there.
The Kitfox does what it does very well, because it is what it is.
To add 2 seats is not just enlarge the cabin, then you have to reinforce the fuselage, then you need more wing to carry the increased weight, then you have to add more HP to carry the increased... it just keeps snowballing without the results you want.
Ralph