Congrats in the purchase. Sure she will love it and you will probably find yourself trying to pinch it for a spin too.
There is a dedicated XR4 forum which has a heap of info but in general the main areas which are restricted from the factory are:
Intake
Flexi - Hitech Exhausts or Milltek
Cat - Milltek
Resonator and Back muffler - HiTech or Milltek
Tune - Dream Science or Bluefin
Suspension - Eibach springs and Whiteline Rear Sway Bar
Pretty easy to free up a bit more power and make them handle like a go kart.
Have fun
