Book Four - Duet is the fourth season of the Cartoon Network series Infinity Train and serves as a standalone prequel to Book One. It premiered on April 15, 2021, on HBO Max.
Min-Gi and Ryan are childhood best friends who dream of performing in a band and traveling across Canada, together. After a sudden falling out splits them apart, both find themselves aboard the mysterious Infinity Train. Lost and confused, they meet Kez, a free-spirited concierge bell, who accompanies them on their journey. As tensions rise and old wounds come to light, both Min-Gi and Ryan must decide if they can work together or else risk losing themselves forever aboard the Infinity Train.
Book Four was formally revealed through HBO Max's twitter on February 17, 2021.
The Book Four trailer was revealed on Twitter by Entertainment Weekly on March 11, 2021. Here it was also confirmed that Book 4 will be the final season. Owen Dennis also revealed the season's full title in a tweet. Dennis later explained on Twitter this season was not intended to be the final one when it was written (Dennis and the writing staff claim to have enough stories to last eight seasons), and expressed a desire to continue the show if given the opportunity.
HBO continued to advertise Infinity Train. On March 25, 2021 the poster for Book 4 and a more detailed synopsis (the one above) was released on HBO Max's twitter. Then on April 8, 2021 a exclusive clip was released on YouTube