It is usually no problem having another vehicle sitting under a car up on a 2 post hoist, they are very stable if attached into the concrete properly. There should be no problem with suspension hang down, the only advantage I can see for a 4 post is as mentioned putting assorted items on the ramps but then you still have to remove those items and find another place for them when the vehicle needs to come off.
The 4 poster I used was mainly for doing front end alignments or a quick look around under a vehicle, I had 4 x 2 post hoists and 1 x 4 post hoists in my last workshop.
