I am still rolling a TC4 and I say that if the hobby shops still stock parts for it, keep rollin. For the parking lot racing I do, a TC4 does the job and I can still get parts and im sure TC3 parts are easy to get too.
Most people at our track (paved and dirt oval) prefer the TC3 over the 4, Im sure the 4 and 5 have its advantages racing on road, thats what their intended purpose is