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Yes there are five of them. When you kill all four normal bosses, the door to the king of the island opens.
If you want to explore the complete game world but you don't have enough time to play the game, you can start the game from command line and activate the developer/cheat mode by pressing A+2+Enter after starting a new game. After activating developer mode you will see all possible cheat codes in the command line window. Feel free to explore, hack and mod the game (using edit mode). Such stuff can sometimes be more fun than playing.