To obtain a 4th difficulty, you need to get the highscore on all 4 modes of gameplay.
Zoo keeper: 1200000 points needed.
Tokoton 100: 1800000 points needed.
Quest mode: 10000 Yen needed.
Time attack: 600000 points needed.
After getting all 4 high scores, a 4th difficulty appears in the top line of the option menu.
This difficulty means "Expert" or literally "difficult-player" and has a huge advantage of doubling all the normal points you achieve. This secret was found by mtpfreak and verified by bemani.
September 1, 2005 - 8:35am