Unlockable 4th difficulty, "gekimuzu".

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.