Here's the gist. I don't mean to imply that Sprinter 4x4 is bad, but it's not really a full-on 4x4.
"The 4x4 gets an extra 4.3 inches of lift in the front and 3.1 in the rear. The 4WD engages via a dash-mounted button that sends 35 percent of traction to the front axle. The low range option adds a 40 percent lower crawl ratio. The shortcoming is that whole drivetrain runs through open diffs front and rear. But Mercedes counters that with traction control that grabs the brakes of the slipping wheel to send traction to the tire getting the grip. This works well up until you’re in really steep, deep mud, whereupon (for our convoy, anyway) a Mercedes guy with a shovel appears and digs you out."
One artifact, mentioned elsewhere, is an occasional rumble strip sound from the 4x4 transmission.