Left:  A poster for the fourth movie.
Right:  The American poster.  Click on it to see a full-size version.

Forty years ago, a young boy named Yukinari hears a voice in the forest.  He remembers being told that if he ever hears the voice of the forest, stay completely still or else he'll find himself in another era.  The "voice of the forest" belongs to the legendary pokemon Serebii, a pokemon who is being chased by a hunter.  As Yukinari goes to assist Serebii, the pokemon decides that the best way to escape is to travel to another time.  The pokemon takes Yukinari with it forty years into the future, to the time of Satoshi-tachi. 

In the present-day, Satoshi-tachi are wandering around the Forest of Ubame when Satoshi spots a Suikun.  Anxious to get a closer look of the legendary pokemon, Satoshi follows it into the forest.  There, he comes across a treetop village where he meets Towa and her grandaughter Miku.  Suddenly, a flash of light is seen coming from the shrine in the forest, so Pikachu runs off to investigate.  At the shrine is Yukinari, the young boy from forty years before.  After he is brought back to the village and is woken up, Yukinari informs the group that Serebii is in trouble.  Satoshi-tachi venture into the forest to find the time travelling pokemon hiding on top of a large tree.  After convincing it that Satoshi and Yukinari are its friends, the two young boys begin befriending the pokemon. 

After a little while of playing, Vicious, an elite member of the Rocket-Dan, appears on the scene and tries to capture Serebii.  Satoshi-tachi manage to ward off Vicious and his giant mecha, hiding deeper into the Forest of Ubame.  Once they're safe, Satoshi and Yukinari play with Serebii and become close friends.  Eventually, Vicious catches up with Satoshi-tachi, but this time he actually succeeds in capturing Serebii with his Dark Ball!  The Dark Ball turns whatever pokemon is captured with it evil, so now Vicious has complete control over Serebii!  Vicious instructs it to start destroying the forest, and the seemingly emotionless pokemon obeys.  Using the debris present in the forest, Serebii creates a sort of vine monster that it uses to attack the forest. 

With a little help from Suikun (who appears after sensing that the forest is in danger), Satoshi is able to reach the summit of the vine monster where Serebii is located.  There, Satoshi reminds Serebii of all the good times they had together and manages to break the spell of the Dark Ball.  Now that Serebii is back to normal, the vine monster falls apart, causing Vicious to retreat.  However, the combination of the Dark Ball's effects and all the energy it used to destroy the forest causes Serebii to become deathly ill!  Satoshi-tachi try to resurrect the pokemon, but it isn't until a group of Serebii transport themselves to the forest (undoubtedly from different eras) to revive the pokemon that Serebii comes back to life.  Vicious makes one more desperate attempt at capturing Serebii, but he is quickly thwarted and left to the villagers for punishment.  With Serebii safe and the forest out of danger, the Serebii transport Yukinari back to his own time.  After Satoshi bids his new friend goodbye, he continues his journey to the next land.

The first word of the fourth movie surfaced on-line, like so many rumors, on a message board.  Someone claimed to have read an interview on TV-Tokyo's site with Matsumoto Rika (Satoshi's VA) explaining what the fourth movie will be about.  The person said that the contents of Uchikido-Hakase's GS Ball will be revealed to be Pokemon #251, Serebii.  The rumor also stated that after the Jouto season of the anime, Matsumoto Rika will quit being Satoshi and that the series will end.  The whole thing was found out to be a big hoax and the fourth movie was blown off as a mere rumor. 

However, at the end of the third movie, a brief commercial for the fourth movie was shown.  Soon information started emerging from various Japanese websites and magazines, and now the fourth movie is a reality.

The movie disproved a lot of rumors that had been buzzing around the Internet.  First of all, the GS Ball is never mentioned in the new movie.  In addition, many fans are expecting Myuutsu to appear in the movie.  It was shown in the preview for the third movie, and most people assumed that the fourth movie would have Myuutsu play some role.  It doesn't--in fact, it doesn't appear at all!  Despite these disappointments, the movie is interesting and is the favorite of many fans.

The fourth movie came out in America on October 11th, 2002 under the title "Pokemon 4 Ever."  The movie, distributed by Miramax this time (instead of Warner Brothers), came in 18th place on its opening weekend, earning $717,061.  That doesn't sound like a lot, but considering the limited release the movie got (it was only shown in 249 theatres in about 20 cities), those numbers are pretty good.  Miramax promised to back the movie up with a fresh advertising campaign to breathe life back into the franchise, but they really didn't keep that promise.  A limited-edition Celebi card-e was included with the price of admission. 

The movie theatres in Japan ran a really nice promotion.  For ¥1400, you got a child's ticket (which is normally ¥800), a Kakuleon card (one of those 3D cards that change when you title it at different angles), a pencil board (you remember in elementary school that really flimsy paper with the really big lines you used while you were learning to write?  Pencil boards go up underneath this paper to keep them from tearing), a Serebii pendant, and a Monster Ball pin.
 

Here's a picture of the 65-page film comic version of the movie, for those who can't find a copy of the movie on video.  This image is from Coro Coro Comic's website.

Thanks to Pokemonica for the above staff list.