Sounds more like the fuel pressure regulator...truthfully, in a fuel-injected system, pressing on the pedal while starting really does nothing...it works because there may be some fuel left that hasn't been backwashed into the system by the regulator.
As far as not being able to rev past 4500 rpms. Are you doing this while in "Park"? This IS a MB design, and the engine management system shuts down a couple of cylinders to prevent overreving the engine with no load...my M104 does the same thing.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car