The Kung Fu Panda 4 trailer sets up how the franchise's three biggest villains can return, and their appearances look quite likely as a result. The first three Kung Fu Panda movies saw Po encounter a handful of powerful villains. His journey as the Dragon Warrior began by fighting Tai Lung in the original movie, before the sequel dove deeper into his backstory with the introduction of Lord Shen. Kung Fu Panda 3 then brought Kai into the picture, giving him the power to steal other people's chi and have kung fu masters fight as his minions.
https://screenrant.com/kung-fu-panda-4-villains-return-setup/