Have to say that a 4 cell mod class would be very good for indoors and there isn't anything to say that we can't have a 6 cell and a 4 cell class. 1/12th have had that before.
There is some interesting comments on the JMRCA thread on 4 cell Touring Cars on Race Chat.
Looking at the amount of stock motors some drivers are getting through then a couple of mod motors a season is cheaper and brushless could be cheaper still!