General answer in principle (but see below for your particular situation): because a concrete slab makes an excellent coop floor, but a lousy run floor unless you have serious rat issues or have no choice *but* to use the slab, I would normally vote for putting the coop directly down on the slab so the concrete is the coop floor. (You'll put bedding on it, of course). As long as that does not decrease the area you have available for a run. If your run space is very limited, probably better to get the extra area by raising the coop up and using the slab underneath it as part of the run space.
HOWEVER.
This only applies when the slab is the same size as the coop.
I gather your slab is much larger than you will make the coop?
Please be aware it can be hard -- real, real hard and not always successful no matter how hard you try -- to waterproof under the sills of a structure built on slab with extra slab sticking out the side. What happens is that when it rains, water pools on the slab and wants to run in under your walls. Your coop then gets real soggy every time it rains, which is Bad.
There are special membranes or sealants you can use under the walls to try to prevent this. However, it's a tossup whether you will have them actually work for you.
My advice is that if you cannot roof the WHOLE slab, it is really a lot better to raise the coop up and avoid this whole issue. This will also make a reach-in coop (as you are apparently planning) a lot easier to access, b/c you can do it standing up. Please mock up your coop with cardboard or something first, though, because 4x8 is awfully large to make a reach-in-only coop and you may find that the only way to reach objects (e.g. eggs, chickens) at the far side/corners of it is to lay on your face in the pooey bedding whcih many do not enjoy doing, so you may wish to smallen your dimensions up a bit
Note that if you raise the coop up it will be tippier and catch the wind more (higher center of gravity) so unless it is in a very sheltered area and you don't get bad storms, think about how you will anchor it down.
Good luck, have fun,
Pat