For each race you can gain up to 16 points for coming 1st, with the amount of points gradually decreasing the lower you place.
To progress to stage 5 you must again gain 48 out of the 64 points available within the four races you can play in stage 4. This can be achieved by either three wins or a variation of placements as long as they bring you to, or over the 48 point mark.
Always aim to perform as many tricks as possible to give yourself an edge or more importantly even be able to keep up with the competition, but make sure you can these without bailing as that will seriously hinder your chance of achieving first place.