Boost Mode is an unused mode for Tekken 4. While the code still exists and can be included into game via hacking, Boost Mode cannot be unlocked legitimately.
When enabled, there will be the word "BOOST" at the bottom center of the screen, and the movements of characters will be buffed to great extremes. Similar to Turbo Battle in Tekken 7, the recovery of attacks except throws has also been decreased, and combos may last much longer and go much farther than normal gameplay.