Volume 4 is the last of the series. The kids discover a new form of Digivolution that involves the Tamers merging bodies with their Digimon (creepy!). This basically stresses the fact that in Digimon Season 3, the kids take a lot more part in battles and don't simply leave all the fighting to the Digimon (the Tamers can use their Digimon cards to Modify their Digimon, for example--swiping a card on their weird Digivice can give their Digimon wings, better speed, lasers, etc. to help them fight better). After some battles and self-discovery and so forth, the kids return to Earth, battle more evil forces there, and eventually save the day. I think at the end they have to lose their Digimon and go back to reality without them...but the children learned a powerful lesson about responsibility and friendship, and the audience wipes a single tear from its collective eye.