From Tekkenpedia

Arena
Walls	Yes
BG Music	The Strongest Iron Arena

Arena is a stage in Tekken 4. This is where Heihachi Mishima is usually fought. This fight is a cage match, as it takes place in a ring surrounded by metal bars. Spectators cheer the fighters on from outside of the ring. Also, outside of the ring, a screen will show you and your opponent fighting.

[edit] Trivia

• In the Japanese version of Tekken 4, there is a way to get rid of the bars around the arena. The player must first finish time attack mode, and must then play story mode (with the same character), skipping Heihachi's intro. When the round begins, the bars will be gone (actually, they're invisible). You can leave the arena via the ramp Heihachi came down on. (This glitch only works with people who actually fight in the arena, so it won't work with Jin.) (The video can be seen here: [1].)