The 4-Matic is an all-wheel-drive system, also called full-time four-wheel drive. The distinction between these and part-time four-wheel drive is the existence of a center differential.

The M-B 4-Matic uses a differential that applies more torque to the rear wheels than to the front, under normal driving conditions. Well, at least in the E class, but I would be surprised if the R were different.