You're going to have to have the GC do something better than just place a a little block of concrete for the lift to sit on... cutting some holes in the floor and placing some random sized blocks of concrete to hold up a car is a little scary...It might seem counter intuitive..that the 4 " think slab is OK but a 12" deep block of concrete is not...that was because the 4 in slab is monolithic... bending and rotational forces are spread out on the slab... won't work with a tiny cube of concrete..
The GC will need to construct some sort of pier or column that will be able to resist any force when a car is up on the lift..remember the load of a car is not evenly distributed on the lift (think a 911)
I just scribbled a crude back up for why a random sized block of concrete is no good... Note that I don't do much drafting or sketching anymore...
Also have the GC pony up for high early strength concrete (not quick set) it has a few more pounds of cement/yard than "regular" concrete, and you will be able to get your productivity back sooner...
