If you haven't sold your NTC3 by now you're not gonna get much for it. I have 2 and I'm pretty much stuck with them.
I wouldn't go looking for a NTC4 just yet. They're having problems with the electric TC4. They're having a hard time keeping up with the belt cars. There's a couple of rumors about AE investigating belt drive. If they do that with the electric car, they will probably do that with the nitro car as well.