You are in a fast SBR in which you need to complete 8 laps of the oval course. However, at the end of each lap, you must turn around and race around the reverse way. In this case, checkpoint 4 and 8 is when you need to turn around again (checkpoint 4 is half of what the scenario calls a "full" lap so make sure to TURN AROUND AT CHECKPOINT 4 AS WELL AS 8!)
The opposing SBRs are much more cautious and is not very fast around the corners.
Try not to hit the concrete barreirs with the back of the car as it can damage the engine.