[ShadeTreeMech]

From Multi-Displacement System - Wikiautos

Multi-Displacement System
Chrysler's Multi-Displacement System (MDS) is a variable displacement technology which is capable of deactivating four of a V8's cylinders under certain driving conditions, such as when the throttle is fully closed or when the vehicle is cruising at highway speed. The system is used on Chrysler's 5.7-Liter HEMI(R) V8 engine and is available on all Chrysler, Dodge, and Jeep models which utilize this powerplant. These models include the Chrysler 300C, Dodge Charger, Dodge Magnum, Jeep Grand Cherokee, Dodge Durango, Dodge Ram, Jeep Commander, and the Chrysler Aspen sport utility vehicle.
The purpose of MDS is to increase fuel economy. Chrysler states that its MDS technology can increase fuel economy by up to 20 percent. For example, the 2008 Chrysler Aspen is rated at 13-mpg city and 17-mpg highway with the standard 4.7-Liter V8, which does not feature MDS. Upgrading to the larger and more powerful HEMI engine with MDS brings mileage estimates up to 14-mpg city and 20-mpg highway. This provides buyers with improved fuel economy, more power, more acceleration, and increased towing capacity.
The operation of the MDS system is automatic and seamless.

I suspect the reason a 4 cylinder with the valves left closed on 2 cylinders won't run is because the air inside is trapped and acts to destabilize the engine due to 2 shock absorbers effectively being strapped to the crank. The air flowing through would likely cost less energy than it just sitting in there causing issues with the ECU not getting the amount of airflow it expects.