There are other choices besides the two-post and four-post. The low-lift, like
Kwik Lift, which is a lower, cheaper, no-220-required, you can't park anything under it four-post. There's also what I'll call the scissors lift (don't have any ready examples, though I saw at least one on this very board), which is nearly ideal for the 914, but not so ideal for anything else. These lift the car from the four corners of the 914 tub (the donuts), leaving the wheels and the driveline dangling. If you have to work on the exhaust on a front engined car, this model would suck.
The other factor with a four-post IF you intend to do the up/down parking thing is ceiling height in the prospective garage. My garage is 115" from the floor to the joists, which is not quite enough for me to park a Mini and a 914 over/under on a four poster (about 1" short). If you're only ever going to park really low cars on it, no problem. The Mini is pretty short, though.
On the concrete depth requirement for the two-poster; since the thing is going to be bolted to the floor, you could certainly have a contractor come out to dig and repour thicker sections under where the lift will be bolted. You don't have to do the whole floor, just where the posts sit.