Likely you flashed the BIOS into a 1.1 version Xbox, and it's FAR from the latest (which is 4977). 4973 is relatively "old" and only boots version 1.0 Xboxes. If you do not understand the different versions of Xbox, then educate yourself.
If you don't know what you are doing, DON'T DO IT.
Read - read the tutorials, read the various posts, get familiar and COMFORTABLE with modding in general and above all, understand exactly what it is you are trying to do and how EXACTLY you need to do it.