Cheat code warning
Do not use cheat codes if you want to get the Quake 4 Achievements. Once a cheat code is active the game will disable all Achievements. If you have this problem on your saved game files, you must delete all of your current save files for the game and start again.

Refill Ammo
During gameplay, press Back to enter the objectives window, and then enter the following code to unlock the corresponding cheat: Press B, A, X, Y, Left, Right, Left.

If you entered the code correctly, the cheat will take immediate effect.

Invincibility
While playing a game, press Back to display the objectives window, then quickly press Down, Up, Down(2), Up, Right, Left, Right, Y, X(2), A. Your health will still decrease, but will never reach zero.

Get all
While playing a game, press Back to display the objectives window, then quickly press Up(2), Down(2), Left, Right, Left, Right, B, A.

Full health
While playing a game, press Back to display the objectives window, then quickly press B, A, B, A, Up(2), Down, X.

Let me know if any of these cheats do not work. Also, if I am missing any cheats, please contact me and let me know.