Once you've got as far as Kalebo 3 (Gadgetron HQ) you'll find a hoverboard track. This is the ones you need. In order to score points, you need to hold down the left stick and press either R1, R2, L1 or l2. The more you do the more points you score.
If your having problems getting 4,500, going off the track before the first ramp that goes over water will put you back a fair distance, so build up points coming up to it, land in the water, rinse and repeat until you have 4,500. Then just fininsh.