First download Notepad ++ , Then right click on iw3mp file and click on edit with notepad ++.
this will open notepad++
After this on top press on search then click on find
in find space write sv_cheats
press find next
the line sv_cheats will be displayed then cut "v" in sv_cheats and change it to "j"
the line will look like this " sj_cheats "
after this tap on file on top left corner and press on save.
open cod4 mp file
and on console write /devmap mp_crash
choose a class and play for a few seconds
then quit that server and connect a online server and in console write /r_fullbright 1
this will enable fullbright and if you want to change your Field of View write /cg_fovscale 1.(any Number between 1-10)
There you go but whenever you open iw3mp you must do the devmap command or else the fullbright wont work