Is Urusei Yatsura Movie 4: Lum The Forever hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Movie · 1986 · 1 episodes · PG-13 - Teens 13 or older
Action, Adventure, Comedy, Drama, Romance, Sci-Fi, Shounen
Strange things begin happening in Tomobiki Town... When a great cherry tree, "Tarozakura" is cut down while Lum & Co. are making a movie, Lum loses her horns-and her powers! Thus begins the strangest and most lyrical of the Urusei Yatsura movies... (Source: AniDB)
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