Big Head Mode
At the main menu, select Fight Now, then press the following combination of buttons: Left, Right, Left, Right, Left, Right, Left.
100% success rate. 

Unlock Big Tigger
At the main menu, select My Corner, then "Record Books", then "Most Wins-Boxer". Press Up twice and you will now be able to fight as Big Tigger.
100% success rate. 

Unlock All Venues
At the main menu, select My Corner, then press the following combination of buttons: Left, Left, Left, Right, Right, Right, Left, Right, Right.

Mini Fighter
At the main menu, select Play Now, then press the following combination of buttons: Left, Left, Left, Right, Right, Right, Left, A.

Go to: Fight Night 2004 Info Page | nGaming Home Page.