Unlock World 4 in Super Mario Bros. DS

You may have wondered why world 4 won't ever unlock. It's pretty simple and this article will show you how to unlock it.

Steps

Go to world 1, level 4.
Find the Mini Mushroom and become Mini Mario. Remember the ? block where the Mini Mushroom came from, since you'll return to this spot later on.
Beat the level, while in the form of Mini Mario.
Play the level again, and get the Mini Mushroom power-up. Make sure that it's in the bubble in the bottom right corner; this means that you've saved this power-up for future use. Defeat the level again. You may now become big but don't replace the mini in the right corner
Go to the final castle in World 2, and clear the level to get to the Giant Worm boss stage.
Touch the Mini Mushroom power-up that you've saved earlier. Turn Mini, and defeat the worm.
A cut scene will play, were Mini Mario will fall into the mini hole, and walk past the sign. It should reveal world 4.

Try not to jump to much before the worm comes up because you will often land on it.
When defeating the worm, stay close to edges because it usually doesn't pop up in the edges.
While jumping on the worm, press down to Ground Pound onto the worm.
Jump on the worm twice with the normal Mario and then use Mini Mario and jump on the worm the third time pressing down to the ground.