Assuming you have already used the brightness setting in the menu there are two other commands that may be of use. 

r_gamma
Do not set this too high or colours will become washed out. Set between 1 and 1.4, adjust r_lightscale for settings higher than 1.4

r_lightscale
Set between 2(default) and 4, only use when you feel r_gamma and r_brightness have not helped.

You should add these to a file called autoexec.cfg in your Quake4 directory or set using the console. See the FAQ How do I access the console? for details on how to enable the console.