Where are all the boulders at that you have to fast plant i can only find 3 of them, someone help me find all 4 thats the last thing i need to do to beat the game.

I got stuck here too!
From your starting position look around using L1! You should be able to see 3 boulders right....so wheres the last one? Simply ride towards the closest boulder to you, but instead of jumping down to it, continue along the path, and the title of the challenge should come up, illustrating that its close by!!! It is...its very close! These a small path on your left hand side, and the boulders right there!!! Hard to miss when youre going in the right direction!!! Hope this helps!!!