World D-4 is the fourth and final level of World D in Super Mario Bros.: The Lost Levels, as well as the final level in the game overall. Like with World 8-4, lightning flashes in the background of the level and the level's title card in Super Mario All-Stars.
The player starts in a room similar to that the one at the start of 8-4. The player has to jump across a pit of lava while avoiding a long Fire Bar, then while avoiding another Fire Bar at the end of the corridor, the player can find a power-up in a Hidden Block against the wall. Jumping across a lava pit with a Lava Bubble jumping out and avoiding the next Fire Bar, the player then has to bounce off of a Koopa Paratroopa and avoid pipes with Piranha Plants, along with Cheep Cheeps that jump out of the lava until the player reaches the next hall. After a falling lift and a Fire Bar, the player will find a Warp Pipe at the end of the hallway.
The next area is an overworld area, with the first enemy being a charging Hammer Bro. After the second pipe with a Piranha Plant, wind will begin blowing. The player will next encounter two Koopa Paratroopas, two Bloopers, another Piranha Plant, and a hidden Poison Mushroom before a flight of stairs, which the wind will stop blowing beforehand. After a jump across a large pit, the player will find a Warp Pipe with a Piranha Plant.
The fourth and final area has the player return to the castle, where they will encounter a fake Bowser's Brother. Above him is a row of ? Blocks, with the fifth from the right containing a power-up. The next Warp Pipe with the Piranha Plant and Bloopers surrounding it will take the player back to the start of the level. After the pipe is a Hammer Bro, followed by a narrow passage that the player will have to crouch and slide through if they are in a Super or Fire form. Using a lift to get up over a ledge, the player then has to time a fall into a narrow passage while avoiding a Lava Bubble. Finally, the player will encounter a fake Bowser, who throws hammers and has a Fire Bar above him. Upon defeating him or touching the axe, the player will save Princess Peach and complete the game.