Level 4 is the last level in the game, and by far the most difficult. The level is a recreation of the original Donkey Kong, where Mario fights Donkey Kong by going up girders and dodging barrels. Jumping in this level works differently than in the previous 3, as the player can only use boosts, no jumping. These boosts however function more like an actual jump rather than just flying upwards. You boost to get a small amount of height and you go back down, no ways to fly or keep yourself airborn in this level.

There are two ways to beat this level, the first is to play the level as intended. This means jumping and dodging barrels, and touching the ladders so they teleport you to the next floor, however the barrel movement is unpredictable and the ladders are unreliable. The next way to beat this level is by ignoring the ladders all together. If pressed quickly in succession, the player can use two boosts at the same time to get double the height. If performed correctly the player can simply jump up to the next girder rather than using the ladders. However this method is difficult and takes a lot of practice in order to do.

Once the player reaches the top of the girders, its time for the face off against Donkey Kong. The best way to deal with him is to jump onto his platform to grab his attention, hide behind the conveniently placed barrels for about 2-3 seconds, then jump back down to the girder one floor lower. By grabbing Donkey Kong's attention and by jumping back down, his AI may send him flying into the abyss, and he will not respawn. This may take a couple tries so be patient. Once Donkey Kong is dealt with, all thats left is to make it to the back end of Donkey Kong's arena, push over some of the oddly placed barrels, and grab Phineas to complete the game.

See video below to watch level 4 in its entirety with the Donkey Kong fight

Go to 6:20 in the video for specifically Level 4: