1. First, you need to collect all the keys of Pluto Temple in Rainbow Island. * * There are 8 keys, namely green key, purple key, brown key, red key, orange key, yellow key, gray key and blue key.

2. Then, after collecting all the keys, go to the gate of Pluto Temple on Rainbow Island.

3. Finally, use the collected 8 keys to open the door of the temple. You can go to the fourth floor after entering the temple.