In every discussion about "which car to buy" most people tend to chip in for the car they own, so if you wait long enough you will get as many proposals as there are cars on the market
stick to your tc4. Practice, set it up properly, maintain it well and you'll be just as fast with it as you'd be with another car. Once you've learnt the ropes, THEN you can upgrade, and even then you might not be any faster than with the Tc4.
Re chassis stiffness - this is one of the subtleties than 99.9% of the drivers would be totally unable to master properly. This is theory, and marketing. Don