Hey there, welcome to JG!
I'd suggest you hop over to wjjeeps.com and check that site out. It will show you how to go about replacing the front bumper, what parts you will need (right down to the clips) and also the part numbers too.
However, the 2004 WJ is a face-lift model and has a completely different front end compared to the 1999-2003 WJ (2005> is the WK Jeep, totally different too) so you will really need to track down or order up a 2004 bumper.
If your fog lights and grille need changed too you will need to get the '04 specific ones also.
Good luck on sourcing the parts and getting everything fitted!