This stage is Ant Trooper Hill (4-1). This complete walkthrough will provide you with a 100% video playthrough and a screenshot of where you can find all the green stars. We will also note a secret invincible star to help you pass an area with lots of ants enemies.
Game Play Walkthrough Video
Green Star 1
After you exit the first cave, head to the right and climb up the platform with Cat Mario.
Green Star 2
When you first encounter the giant ants, there is a green pipe that you can get into. You will need the Cat Mario to climb and grab the second green star. Remember to avoid the pesty ants that get in your way.
Green Star 3
Bounce on the giant ants and you will get the third green star. You won’t miss it but you simply have to maneuver your bouncing movement on the ant’s head.
You will encounter the first stamp once you get inside the cave. Simply either kill all the ants or just grab the stamp.
Other Notable Secrets
When you get to the little walkway with tons of ants, there is a secret invincible star that you can get. The star will help you pass this part easily.