Laying 4x4 over a cement slab
I have a cement slab (11'x25') which I'm building a wood deck on top and extending it on the ground so the total deck's size will be about 500sq. ft. To make the entry door opening and avoid any steps the height for the joists over the slab is exactly 3.5" (on the ground it's about 8"). I'm building a compound frame consisting of 2x6 over the ground and 4x4 laying just on top of the slab and connecting with the 2x6 over the ground. I'm to anchor the 4x4 but my concern is water and moisture contact with the slab. I thought to use some mastic insulating the wood from direct cement contact but not sure whether it's a pragmatic approach. What do you advise? I probably can insert thin plastic shims between the 4x4 and cement.
p.s. I've been in this project for some time now so things seem obvious to me. Let me know if something is unclear or I missed anything.