RE: TC4 accessories ?
Just to clarify: You can put the Spektrum rx in the Traxxas, but you will need a Spektrum tx. The tx & rx have to match (the vehicle doesn't matter).
Or you can just get another Traxxas rx & put in your car & then use the Traxxas tx for both if you like. I'm not sure which traxxas tx you have, but as long as you can link it you should be fine.
One recommendation for track running in regards to the tx: Look for something with exponent. This allows you to adjust the sensitivity around 0. Meaning, when you get on your straight away, a hair of turn on the wheel doesn't slam you into a wall, but the farther you turn your wheel the more it gives you & it does not limit your steering. Some like straight linear control, but a lot of people use exponent tuning (I do as well). Just something to think about when thinking about which transmitter you want to use.