Hop onto the small platform at Point 1, then jump straight up to find an Invisible Block containing a 1-Up Mushroom.
When the nearby Grinders move away from Point 1, hop onto the used Block and jump up to grab the Star Coin.
The small pipe at Point 2 leads to a hidden area. To use it, you must bring a Mini Mushroom into the level.
As Mini Mario, pass through the pipe, then hit the ? Blocks to find a Super Star. While the power-up is active, perform a series of wall jumps to destroy the Grinders above you.
The second Star Coin is on a platform at Point 3. Stomp the Dry Bones patrolling the platform, then collect the Star Coin before Grinders appear on the nearby tracks.
The third Star Coin is hidden beyond the wall at Point 4. Locate the Invisible Block near the door, then use it to reach the hidden passage.
When the battle starts, Wendy Koopa floods the room with water. Swim away from her as she jumps around the room, and take care to avoid the Cheep Cheeps that emerge from the pipes. Do not attempt to attack Wendy Koopa while the room is flooded.
When the water drains below the floor, stomp on Wendy’s head. Wendy pulls into her shell and bounces around the room. Dodge the shell and the Cheep Cheeps that launch out of the water. When Wendy emerges from her shell, stomp her a second time. Keep dodging enemy attacks and stomp Wendy’s head a third time to end the encounter and unlock World Four.