Some key things to check:

When the screen is black on the Mk4, can you still communicate with it using NavCoder? Read the nav settings, read the nav languages... etc. If you can communicate, then the processor is running and the comms interface is working

Is the Mk4 coded for Mono or Colour? check the label on the top - does it say BMWM01S or BMWC01S? M=mono, C=colour


If Mono, then the video outputs are switched off and it needs to be recoded to colour