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 (or in the case of All Stars the actual 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 (in All Stars, this was replaced by the actual Bowser himself). Upon defeating him or touching the axe, the player will save Princess Peach and complete the game.