A slight slope will keep water from pooling at the base of the tub. It isn't that much of a s;ope so you are probably fine either way.
Slabs that are only 4" thick and have 1" exposed will get water running underneath them (which is what the drainage rock is for) but, being paranoid, I wouldn't exacerbate any erosion with hot tub water. I'd make the slab bigger and have that excess water stay on the slab and evaporate. Assuming the base of the hot tub can handle the moisture.
Depending on where you are, where the water table is, whether your ground is settled or not, whether you are on a slope, blahblah floating monolithic slabs can be tricky. It's good that you are contracting out, but I remember when I shopped around and I got estimates for a 8'x9' slab that varied from $800 to $1600 in MD. The $1600 proposals were technically sound, the $800 were *not* in my opinion.
We would up with 6" concrete with 8" footers, 1/2" rebar reenforcement and a vapor barrier. All over about 2" of drainage rock. Some people put a grounding bar in the slab (we did not). Some also put the conduit in the slab to make things look nicer (but this makes it hard to move the conduit later.
-Ed