The Caterham Styled cars are nice but the superlights dont have competition. Instead of having the 4 and 6, I'd look into some of the other superlight track day cars that are out there. The Atom, The Radical and the New Lotus. These in there own little group could realy make an interesting series. They could be set for either road leagel or race. Ie with or without wings and slicks. A V8 Caterham would never keep up with the DTM style cars in S2 but a superlight is quick and nimble in its own right.
I am not suggesting the addition of the other superlights mentioned in the post, nor am I suggesting that this LX8 should be included. I am just makeing the point that the LX4 + LX6 are in a group of there own yet not competitive with each other.