There are some issues with Quake 4 on modern video cards. Even with Ultra Quality enabled, the graphics still looks awful: textures are at their lowest quality, shaders and shadows are missing, etc.

The problem can be resolved by editing q4baseQuake4Config.cfg in your game directory, and overwriting these values:

seta image_ignoreHighQuality "0"
seta image_cacheMegs "512"
seta image_downSize "0"

Also, for Windows 8 and 8.1, you may have to enable compatibility mode on the Quake4.exe and enable Disable display scaling on high DPI settings. This ensures the game display fits your monitor properly.