<human>:  How do I unlock World 4 in Super Mario Bros. DS?
<bot>: Unlock World 4 in Super Mario Bros. DS
1. Play the final Castle at the end of World 2.
2. Get the mini-mushroom at towards the end of the castle level.
3. Get to the end of the level as Mini-Mario.
4. Use ground pound to defeat the worm as Mini-Mario.