I'm not sure what you mean by 4200 rpm limiter, I've only heard about the 5000 RPM limit........I'm the one who disproved the RPM limiter with the fuses pulled......youtube video that ended that thread here

http://www.youtube.com/watch?v=RwRK97G2lFM

There was also some discussion that there was a diferrence between model years, that might be what you are experiencing.

As far as entering "dyno mode"......either have your ECU hacked or find someone with the Mercedes "STAR" software and breakout box and it's possible.

I just threw this video in for fun......it's a no fuses and disabled yaw sensor run from behind a Dodge dealership.

http://www.youtube.com/watch?v=R11E_lsY2B0