They make composite shims.

Plastic resin? I Know the yard tractor weighs about 500 pounds, various other large equipment probably another 800 pounds plus miscellaneous stuff in the lofts etc. another 1000 pounds. The shed is heavily constructed, so probably weighs over 3000 pounds. So may be just under 10000 pounds total. While the temperature rarely get to less that 10, summer temperature are routinely in the high 90's.

Will resin support that weight at those temperatures?

I regularly use composite decking like Trex as shims. I've bought inexpensive plastic cutting boards at the local dollar store, cut them up and used the pieces as shims to isolate wood from concrete.

What's wrong with treated wood?

It does not seem right to put a relatively thin piece of wood treated or not between that 4X4 and the concrete block that is only a couple of inches from the ground in moisture prone environment.