From the Super Mario Wiki
World 4 in Yoshi's Island DS 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.