Haven't we had this conversation before? The 'Touring' in the Super Touring category is not, and was never meant for traditional 'touring cars' (I think of the BTCC in it's heyday). It was created for a destination for World Challenge cars (in both categories) to land and otherwise to be built to the rules. Says so right in the Purpose statement of the class. Heck, 'Improved Touring' has nothing to do with 4 door saloon cars either.

Be careful what you wish for here. People largely build and race what they like. You create a class for 4-door (assuming your interior spacial numbers necessitate that) you are really limiting your audience IMHO.

Maybe it's separating the STL class into STL-SC (sports car) and STL-TC (touring car) that you are driving at?