In L4 you had twice the traction you had in L2, since the front diff was now engaged. However, when you are climbing dunes the thing you need most is momentum- if you have enough of it, you don't need as much traction and if you get it right, you can get to the top of the dune much more easily.
The only way to learn how to get momentum right is to join as many drives as you can, and to listen to, and watch, experienced drivers.