4C has to do with the video card in the NAM. A dipswitch you may have flipped on or off. (position 8 I think, it's in the manual)
The 5A I think is a setup question the computer is asking you.
Make sure you have the dipswitch for monitor package off, or plug in your monitor and see what the issue may be. If the computer has a question, it will freeze until you answer it, go into the BIOS and fix the problem, etc...