I don't know much about golf carts but I do know about construction. Typical 4" conc slab will have a strength of 2500 psi@ 30 days cure. You are correct to consider the material below it a good base should be compacted to 90% (that is hard as the hubs of he11) because if the ground under the slab settles well, the concrete is not gonna bridge it for long without cracking and settling also. Is the lift stationary or mobile? Pinpoint loads should be beefed up. A lift with a cart on it is okay on a gd 2500 (or 3000# psi) floor but, a car or truck on a lift should be a thicker floor. Consider 6" with reinforcing wire or fiberglass reinforcing threads in the conc.
What is your ground like and what are your typical frost levels? All these are factors to be considered...
