Hi, and welcome to the slippery slope of upgrading!
I don't have a TRX4, so won't comment on any specifics in relation to the model, but my general advice would be to upgrade as it breaks. I was too quick to try and upgrade my Element enduro, and I probably spent more time worrying about what upgrades to get instead of enjoying it!
Usual areas to upgrade would be
- lowering the centre of gravity. Adding unsprung weight low down is a good way.
- Tyres. I believe the traxxas tyres are quite capable, but depending on what you want to do with it, you could look at softer compound tyres. I have some proline BF Goodrich mud terrain in predator compound and they are very sticky!
- ESC & Motor. There are lots of options, whether that be brushed or Brushless. I have a HW 1080p Esc paired to a Holmes Hobby Crawlmaster sport in mine, and it does great for what I need.
Be sure to post some pictures too!