Concrete slab for hot tub
I don't get the part about the two piles. Is this a typo, or did you drive two piles into the ground to support the slab? As for support, a 4 inch slab should work fine provided it is properly supported by a suitable crushed stone or clean gravel base on adequately compacted and prepared soil. The concrete will be entirely in compression, which is the way concrete is strong, unless you have voids or compressible soil supporting the concrete, in which case you will have serious trouble.