The ground clearance gained by the 4x4 isn't much from what I read. AFAIK, it is at most an inch or two, so the CG difference is not that much. For a van that lives its life in a city, I'd not bother, but for snow and general handling, I'd definitely consider this.
Mercedes has had 4WD vehicles for a long time, so it goes without saying that their setup will be as good as you are going to get.
I am sort of surprised it isn't AWD (where the vehicle runs 2WD, but powers all wheels if it senses slippage, similar to the Honda Ridgeline.) However, having 4-Low can be quite useful, even if one isn't going far off the beaten path.
(Edited for clarity, to reflect the difference in height by the 4WD mechanism as opposed to the van's clearance height.)
