Hi. Great discussion. I feel that if it is a drive-on 4 post lift, then it should have two sliding/rolling jacks between the drive-on ramps. Many lifts offer one jack, if any, typically a scissor-style or rubber air spring design, but a second one is a pricey option that many opt to go without. You really want to be able to jack the whole vehicle in the air (to send out the tires, for example). A drive-on lift is fast for point-and-shoot driving on, and is arguably the best for truck oil changes. It can be a little more cramped under there because you have to work in the space between the relatively wide ramps. If you're working solely on big trucks, you can make a strong argument for this lift type. The DOWNSIDES to this type of lift are: (1) It takes up way more space in the bay and has a huge footprint. If you have a huge building, it's a non-issue, but if it's cramped as it is, consider that. (2) A drive-on lift has long, hinged ramps that protrude from the rear of the main ramps on the lift. These hinged ramps allow the vehicle to drive up and onto the steel main ramps. The ones I've seen/heard make a loud banging noise every time a vehicle drives on/off. Worse, when the lift is up, these ramps swing downward and can block the space below the lift enough that a second vehicle cannot easily be parked under the raised vehicle. If the bay that's getting the lift doubles as an everyday parking space, it's worth thinking about the jolt up and down as the car goes on and off.
Personally, I have a 30' bay w/a 12' 6" ceiling and an Overhead garage door that hugs the ceiling as tightly as possible (to clear a raised vehicle). I have a Rotary asymmetrical 10,000 lb 2 post lift. I paid about $6 grand, including installation and wiring (didn't have to lift a finger, you should really let a pro do it; It's so worth it). It sits on a 6" floor. They say 4" is minimum. I don't know if I could trust it on just 4", though. What I really like about my setup is that the 2 post takes up minimal space, raised or lowered. I use the bay with my lift as my daily parking space, when possible, and I like just driving in and out with cars OR motorcycles. You can also "stack" 2 cars easily with a 2-post, since there are no ramps dangling. I know there are 4 post lifts made for vehicle storage, as well. This often makes my 3 bay garage into a 4-car garage. My 2-post will lift a crew cab, but it starts to feel like it's at it's limits and if I was working on mostly trucks, I would definitely go for something bigger. They make bigger 2-post lifts. Dodge/Freightliner/MB Sprinter vans require a trick setup with long-reach arms.
Hope this was helpful.