First, is the ENTIRE concrete slab going to be contained in the coop. If not -- if some of it sticks out around the edges -- then you can expect to have water coming in under the walls as a perpetual problem. There are products you can buy to put under the sills of the walls to try to prevent this but BYC users have had very mixed and IMO rather unencouraging results with 'em so I would not hold my breath.
Really, if the concrete is wet, IMO the best solution is a) if you won't be covering the entire slab with the coop (or with a roof, anyhow) then make a raised coop so there is at least 8" and preferably more like 14"+ of open space underneath it; or b) if you WILL be covering the entire slab with the coop (or with coop plus roofed run) then prep and sealer-treat the concrete and you will be good to go. In the latter case, you would either build on p/t skids if it's intended to be a movable coop, or more usually you would simply do normal stud-wall construction, ideally shooting a coupla bolts thru the sill to anchor it to the slab in case of wind.
A plywood subfloor on 2x4s is likely to become a mouse farm as soon as some lucky mouse finds its way in (they only need the tiniest gap...) and unless you used ground-contact-rated p/t 2x4s and marine plywood I would expect it to start to rot and soften within a few years, probably less than five anyway. If you've already built it and just with normal lumber, I would suggest that if you want to get some reasonable lifespan out of this thing, it'd be highly worth the aggravation of jacking the whole coop up, putting suitable-thickness floor joists underneath (I don't know what size your coop is, but 2x4 is likely not to cut it, at least not if you want longevity and structural soundness), and putting it on cinderblocks or treated-wood supports or something like that to ensure it is a reasonable height above the slab. Then rig some form of anchor so it can't blow off.
JMHO, good luck, have fun,
Pat