Quake 4

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

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

Full ammo for all weapons:
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While playing the game, press Back to display the objectives window, then
quickly press B, A, X, Y, Left, Right, Left.