Stage 4 (Donkey Kong Jr.)
From the Super Mario Wiki
Stage 4 is the fourth and last level of the game Donkey Kong Jr., and the second part of Mario's hideout. This level also appears in a microgame in WarioWare: Twisted! and WarioWare Gold, though in the latter, it appears in level 3.
Mario is on top of a tall building with Donkey Kong and Donkey Kong Jr. has to, similar to the 100m stage with the rivets, move all of the keys into their keyholes while avoiding the Nitpickers and Snapjaws. This time, there are more Nitpickers, but they do not bombard Donkey Kong Jr. with eggs like in Stage 2. After moving all six keys into their keyholes, the platform (and cage) disappear and Mario and Donkey Kong with both fall down. The game then ends and restarts to the first stage of the game.