This is a skill point.
To reach a score of 4500+ you will need to do the best tricks as often as possible. To do tricks you must hold in any direction and press the shoulder buttons ( ) in any order. If you do the same trick twice in a row, you will get less points from it, so mix it up as often as possible. To make sure you get the 4500+, you can purposely run yourself off the track near the first ramp that jumps you over water. This will reset you back before jumps so you can continue doing your tricks.
Your score is tallied at the bottom of your screen, so you know how far off 4500 you are.