From the Super Mario Wiki
World 4 is the fourth world in Yoshi's Island DS. It takes place in the mountains of Yoshi's Island, becoming snowier and colder in the second half as Yoshi scales greater heights. In World 4-5, Baby Bowser encounters Yoshi and his party, and decides to join them on their adventure. However, this alliance is short-lived, as at the end of the world, Baby Bowser abandons the party as he attempts to go off with Kamek. World 4, like all other worlds in the game, has a total of ten levels; six regular, two hidden, and two bosses. The bosses in this world are Six-Face Sal and Big Guy the Stilted.