Create a shortcut to the Quake4 executable and edit the shortcut's command line with the suffix +set com_allowConsole 1 to enable the console. Note if the command line uses quotations, add the suffix outside of the quotes.

While playing, press CTRL + ALT + ~ to drop the console (or just tilde) and enter the codes as you like:

god ... Invulnerability
undying ... Player stops taking wounds at 1 health
noclip ... Clipping toggle
give all ... Stocks all weapons and items
give armor ... Stocks body armor
give health ... Stocks life/health
give ammo ... Stocks ammunition
Spawn ... Spawn command used in conjunction with a target

• Spawn weaponmod_hyperblaster_bounce2
• Spawn weaponmod_lightinggun_chain
• Spawn weaponmod_machinegun_ammo
• Spawn weaponmod_nailgun_power
• Spawn weaponmod_nailgun_seek
• Spawn weaponmod_nailgun_rof
• Spawn weaponmod_nailgun_ammo
• Spawn weaponmod_railgun_penetrate
• Spawn weaponmod_rocketlauncher_burst
• Spawn weaponmod_rocketlauncher_homing
• Spawn weaponmod_shotgun_ammo

listcmd ... List all documented commands
listcvar ... Dsiplay all cvars
listentities ... Display all Q4 entities
listmonsters ... Display all Q4 monsters/enemies