The four final crowns are located at the tops of four towers:
TOWER - LOCATION Tower 1 - North of Horne, and the Witches manor. Tower 2 - Look on the map, in the water area, south of Invidia and north of Tower 1, there is Tower 2, you'll have to take your ship to get to it. Tower 3 - Located in the town of Liberte, on your boat at the northern most part of the town Tower 4 - Located on the landmass in the north-east around Spelvia.
Every tower is locked by gold locks that take a key that you receive from Rolan late in the game.
The four secret crowns are:
Ninja Special Ability: Encounter fewer enemies The abilities you'll learn as you level up the crown allow you to better evade your enemies.
Sage Special Ability: Enhance the effects of white and black Magic The abilities you'll learn as you level up the crown will allow you to lower the cost of Black AND white magic by 1
Scribe Special Ability: Grow more powerful the more time you spend playing The abilities you'll learn as you level up the crown will be based on time damage dealt by the time you've played, healing allies health based on crown levels, etc...