Just trying to offer some advice here as I have been down a similar road that it seems your are about to follow. I had a beginer car, I spent almost $300 on the car RTR and I spent anothe $1000 trying to make it as fast and handle as good as a $400 NTC3. It still was never that caliber of a car, sure it was a great car and I had fun and learned so much but if I had it to do over again I would have rocked it stock with very few upgrades and then spent the money on a race ready car.
A race kit is anywhere from $350-$600 and you can see I well surpassed that trying to get and enty level car to be a compitition car. My advice is to have fun with your 4-tec, they are known to be fast and to handle corners poorly so no mater how much money you really spend on it that will be it's weakness. Start looking at cars like the Trinity Reflex, Mugen MTX-3 and HPI R-40, and Serpents look at the design and layout. you will find them all to be very close in design. Low center of gravity, weight distributed as even as you can get it. They are very light weight and very adjustable right out of the box. You should be seeing better traction with foams as someone else already stated so that would be my first clue.