I would use some pieces of flat stainless steel or aluminum approx 1/8" or 3/16" thick to get that wood up off of the concrete slab to allow for ventilation and prevent water from wicking up from the slab.

I wouldn't use mastic. That will only wick the water up just like the concrete or itself. You need something that is impervious.

The other thing you're going to have an issue with is warping/twisting of the 4x4s. You will have to anchor it to the concrete every three feet or so minimum otherwise you're going to have a mess.

numerous deck builders have to me me that the treated 4x4 is designed for water contact. although ideally to prevent it in my case I have little choice. i have no gap to provide real space but I'm assuring ventilation under the deck and keep the incline so water is not supposed to stay there. I'm anchoring 12' long 4x4 by 4 5" lag screws each (so it's roughly 3.5" distance). It seems quite secure. I'll post pictures later when I'm done with it. Drilling 2" long 5/8" radius holes in concrete is no fun...

thanks for your input!
-Roman

Sure, they can get wet, but it's helpful for them to dry evenly. Without elevating them, the bottoms of the 4x4's will always take longer to dry than the tops. There can be some issues down the road, esp. with dirt accumulating.