Only 4 high? that's ridiculous, I've used the orange stacking blocks 6 or 7 (or more once in a while) high for 25 years, still works great. I'm still using the ones I bought in the early 1990's. I find them much more stable than wood blocks of any kind because wood warps and is rarely flat as a board (pun intended).
Stacking blocks are perfectly square and level and don't rock. They will also dig down into the dirt or gravel a little as they aren't solid on the bottom, wood blocks can't do that. The stacking blocks lock together, no sliding, wood can't do that, and screws can loosen and even break as the wood warps. Stacking blocks are also a lot lighter and easier to pack than pieces of lumber, stay cleaner too.
I turn the points to the front-rear, side-side to further reduce any chance of rocking.