From the Super Mario Wiki
Klepto is a character in Mario Party Advance. He lives at the Klepto Ruins and guards the treasure there. He is encountered alongside Mr. E, who entered the ruins in order to discover the treasure's ruins. Klepto is asked by Mr. E about the treasure's location, and flies up to reveal a treasure chest under his body. He then says that the treasure has a four-digit code, and that the code is his "favorite word." The code is the same word he stutters throughout the conversation, for, and therefore 4444. If the code is entered incorrectly, Klepto berates the player and bluntly says that "for mean four." If the code is entered correctly, the chest opens to reveal a giant pile of shiny treasure, and Klepto congratulates the player. Klepto offers the treasure to Mr. E, but he refuses as it is not a mystery any more and quickly runs away. Klepto again praises the player for solving his riddle and gives them the Map Maker Gaddget as a reward. The ending reveals that Klepto changed the chest's code in order to better protect the treasure.