With the mystery solved, Keita and his friends celebrated their victory. As they looked up at the city, now teeming with Yo-kai, they knew that their adventure was only just beginning. The Yo-kai Watch 4, once a simple tool, had become a key to unlocking a hidden world of wonder and excitement.

As Keita began to explore his city with his new watch, he stumbled upon a cryptic message on his phone. The message read: "Purapura ROM: decryption required." Intrigued, Keita showed the message to Whisper, who seemed equally perplexed.

Finally, after overcoming numerous challenges, they reached the heart of the city's digital core. With the help of his Yo-kai friends, Keita managed to decrypt the Purapura ROM.