If you're looking for anime similar to Digimon Season 4: Frontier, you might like these titles. All recommendations are made by Anime-Planet users like you!
Throughout his life, Oozora Reiji has always been rather half-hearted. Never able to fully dedicate himself to anything, he lives a sloppy day-to-day existence, until his classmate Yukino introduces him to Dragon Drive -- a virtual reality game. While Reiji garners a passion for the game, it would seem that its founders have other things in mind than simple entertainment. During a special training, Reiji discovers that Dragon Drive is more than just a game, and now must fight for the survival of the dragons as well as all of mankind!
Digimon Season 4: Frontier and Dragon Drive are both about a digital world where you acquire a digital pet that you then use to battle various other creatures and players, allowing the creature to evolve over time. If you liked one the other should be right up your ally.
both animes involve people getting to fight against evil after being drawn into strange world
Hibari Hanakogenei is a typical twelve-year-old girl who wants nothing more than her very own cyber pet Pata-Pi. One day, her wish is granted when a Pata-Pi appears to fall from the sky into her arms. Hibari names him Densuke, but soon she discovers he's no ordinary Pata-Pi! When Hibari is threatened by an exceptionally large-breasted masked woman and a gaggle of homunculi, Densuke mysteriously transforms into a magical girl and vanquishes the foes. Alongside her friends Suzume and Tsugumi and their own transforming Pata-Pis, Hibari forms a team to help fight evil. Watch out, bad guys! The Cyberteam is ready for action!