To unlock the M4 Carbine which is a gas-operated, air-cooled, selective fire, magazine-fed, shoulder-fired weapon with a telescoping stock you have to earn a 'Silent Assasin' ranking on ALL of the St. Petersburg missions up to and including 'Invitation to a Party' without any aggression. You must pass 'Invitation to a Party' mission last to recieve the M4.

Enter any of the following codes during gameplay...

All weapons:
Press R, L, Up, Down, A, Up, X, A

Bomb mode:
Press R, L, Up, Down, A, Up, White

Full heal:
Press R, L, Up, Down, A, Up, Down

God mode:
Press R, L, Up, Down, A, R, L, Black, White

Lethal charge:
Press R, L, Up, Down, A, Black, White

Level select:
Press R, L, Up, Down, X, Y, B at the main menu.

Megaforce mode:
Press R, L, Up, Down, A, R(2) Restart the level to turn this off.

Nailgun mode:
Press R, L, Up, Down A, White(2)

Punch mode:
Press R, L, Up, Down, A, Up(2)

Slow motion:
Press R, L, Up, Down, A, Up, L

Toggle gravity:
Press R, L, Up, Down, A, L(2)

Unlock Sawn-Off Shotgun:
Complete a level with a 'Silenced Assassin' rank twice.

Unlock Silenced Ballers Gun:
Complete a level with a 'Silenced Assassin' rank.