Open console by pressing [~] and then you might type in:

Gameplay:

allammo 999                            - Ammo for every weapon
allweapons                             - All weapons (fully armed)
loaded                                 - All weapons
suicide                                - Commit suicide
stat all                               - Display all stats
quit                                   - Quits game entirely
exit                                   - Exits the game entirely
playersonly                            - Freeze time (type again to unfreeze)
stat game                              - Game statistics
god                                    - Infinite health
killbots                               - Kills all bots
ghost                                  - No clipping
fly                                    - Fly mode
walk                                   - Turns of Ghost and Fly
teleport                               - You will be teleported to the location on your crosshairs
open (mapname)                         - Open a map where mapname might be:
                                         TUT-BR.ut2
                                         TUT-CTF.ut2
                                         TUT-DM.ut2
                                         TUT-DOM2.ut2
                                         TUT-ONS.ut2
                                         AS-Convoy.ut2
                                         AS-FallenCity.ut2
                                         AS-Glacier.ut2
                                         AS-Junkyard.ut2
                                         AS-MotherShip.ut2
                                         AS-RobotFactory.ut2
                                         BR-Anubis.ut2
                                         BR-Bifrost.ut2
                                         BR-BridgeOfFate.ut2
                                         BR-Canyon.ut2
                                         BR-Colossus.ut2
                                         BR-DE-ElecFields.ut2
                                         BR-Disclosure.ut2
                                         BR-IceFields.ut2
                                         BR-Serenity.ut2
                                         BR-Skyline.ut2
                                         BR-Slaughterhouse.ut2
                                         BR-TwinTombs.ut2
                                         CTF-1on1-Joust.ut2
                                         CTF-AbsoluteZero.ut2
                                         CTF-Avaris.ut2
                                         CTF-BridgeOfFate.ut2
                                         CTF-Chrome.ut2
                                         CTF-Citadel.ut2
                                         CTF-Colossus.ut2
                                         CTF-DE-ElecFields.ut2
                                         CTF-December.ut2
                                         CTF-DoubleDammage.ut2
                                         CTF-Face3.ut2
                                         CTF-FaceClassic.ut2
                                         CTF-Geothermal.ut2
                                         CTF-Grassyknoll.ut2
                                         CTF-Grendelkeep.ut2
                                         CTF-January.ut2
                                         CTF-Lostfaith.ut2
                                         CTF-Magma.ut2
                                         CTF-Maul.ut2
                                         CTF-MoonDragon.ut2
                                         CTF-Orbital2.ut2
                                         CTF-Smote.ut2
                                         CTF-TwinTombs.ut2
                                         DM-1on1-Albatross.ut2
                                         DM-1on1-Crash.ut2
                                         DM-1on1-Desolation.ut2
                                         DM-1on1-Idoma.ut2
                                         DM-1on1-Irondust.ut2
                                         DM-1on1-Mixer.ut2
                                         DM-1on1-Roughinery.ut2
                                         DM-1on1-Serpentine.ut2
                                         DM-1on1-Spirit.ut2
                                         DM-1on1-Squader.ut2
                                         DM-1on1-Trite.ut2
                                         DM-Antalus.ut2
                                         DM-Asbestos.ut2
                                         DM-Compressed.ut2
                                         DM-Corrugation.ut2
                                         DM-Curse4.ut2
                                         DM-DE-Grendelkeep.ut2
                                         DM-DE-Ironic.ut2
                                         DM-DE-Osiris2.ut2
                                         DM-Deck17.ut2
                                         DM-DesertIsle.ut2
                                         DM-Flux2.ut2
                                         DM-Gael.ut2
                                         DM-Gestalt.ut2
                                         DM-Goliath.ut2
                                         DM-HyperBlast2.ut2
                                         DM-Icetomb.ut2
                                         DM-Inferno.ut2
                                         DM-Injector.ut2
                                         DM-Insidious.ut2
                                         DM-IronDeity.ut2
                                         DM-Junkyard.ut2
                                         DM-Leviathan.ut2
                                         DM-Metallurgy.ut2
                                         DM-Morpheus3.ut2
                                         DM-Oceanic.ut2
                                         DM-Phobos2.ut2
                                         DM-Plunge.ut2
                                         DM-Rankin.ut2
                                         DM-Rrajigar.ut2
                                         DM-Rustatorium.ut2
                                         DM-Sulphur.ut2
                                         DM-TokaraForest.ut2
                                         DM-TrainingDay.ut2
                                         DOM-Access.ut2
                                         DOM-Aswan.ut2
                                         DOM-Atlantis.ut2
                                         DOM-Conduit.ut2
                                         DOM-Core.ut2
                                         DOM-Junkyard.ut2
                                         DOM-OutRigger.ut2
                                         DOM-Renascent.ut2
                                         DOM-Ruination.ut2
                                         DOM-ScorchedEarth.ut2
                                         DOM-SepukkuGorge.ut2
                                         DOM-Suntemple.ut2
                                         ONS-ArcticStronghold.ut2
                                         ONS-Crossfire.ut2
                                         ONS-Dawn.ut2
                                         ONS-Dria.ut2
                                         ONS-FrostBite.ut2
                                         ONS-Primeval.ut2
                                         ONS-RedPlanet.ut2
                                         ONS-Severance.ut2
                                         ONS-Torlan.ut2
                                         MOV-UT2004-Intro.ut2
                                         Mov-UT2-intro.ut2
                                         endgame.ut2
                                         ParticleExamples.ut2

Summon items / weapons / vehicles

summon xweapons.redeemerpickup         - Spawns a redeemer
summon xweapons.assaultriflepickup     - Spawns assault rifle
summon onslaught.onsavrilpickup        - Spawns an avril
summon xweapons.bioriflepickup         - Spawns bio rifle
summon xweapons.flakcannonpickup       - Spawns flak cannon
summon onslaught.onsgrenadepickup      - Spawns grenade launcher
summon xweapons.painterpickup          - Spawns ion painter
summon xweapons.sniperriflepickup      - Spawns lightning gun
summon xweapons.linkgunpickup          - Spawns link gun
summon onslaught.onsminelayerpickup    - Spawns mine layer
summon xweapons.minigunpickup          - Spawns minigun
summon xweapons.rocketlauncherpickup   - Spawns rocket launcher
summon xweapons.shieldgunpickup        - Spawns shield gun
summon xweapons.shockriflepickup       - Spawns shock rifle
summon utclassic.classicsniperriflepickup - Spawns sniper rifle
summon Onslaught.ONSHoverTank          - Summon a Goliath
summon Onslaught.ONSPRV                - Summon a Hellbender
summon xweapons.supershockriflepickup  - Summon a instagib shock rifle
summon Onslaught.ONSHoverBike          - Summon a Manta
summon Onslaught.ONSAttackCraft        - Summon a Raptor
summon Onslaught.ONSRV                 - Summon a Scorpion
summon OnslaughtFull.ONSMobileAssaultStation  - Summon a Leviathan
summon OnslaughtFull.ONSGenericSD      - Summon a TC-1200
summon OnslaughtFull.ONSBomber         - Summon a DragonFly/Phoenix

Technical / view

stat fps                               - Shows frames per second
stat none                              - Turn off stats
behindview 0                           - 1st person view
behindview 1                           - 3rd person view
addbots #                              - Add the numbers of bots specified
stat audio                             - Audio stats
FOV #                                  - Changes "Field of View" to specified number (in degrees)
setname (name)                         - Changes player name
setres WxHxD                           - Changes resolution to the specified values (example - 1600x1200x32)
switchteam                             - Changes team
stat net                               - Network play statistics
open (mapname)                         - Opens specified map
setgravity #                           - Set amount of gravity
setjumpz #                             - Set height of jumps
setspeed #                             - Set speed of gameplay
slowmo #                               - Slow motion
togglefullscreen                       - Switches between fullscreen and windowed mode

---

To unlock the 3 hidden characters (the ?'s in the char-selection menu), just open the ut2k4\system\user.ini
and search for

[GUI2K4.UT2k4MainPage]
TotalUnlockedCharacters=

Now change it to

[GUI2K4.UT2k4MainPage]
TotalUnlockedCharacters=Malcolm;ClanLord;Xan

Save & exit... have fun!

Collected with the help of