While a Psycho 4 TV movie was made eventually, it had little in common with the wild, unmade meta version that was supposed to follow the third movie. The original Psycho is essentially the originator of the modern-day slasher, and would later influence the likes of Halloween. Despite being an enormous success, it was produced during a time when sequels weren't the norm. Psycho 2 eventually arrived in cinemas in 1982, over 20 years after the original and after the passing of original director Alfred Hitchcock.
Psycho 2 is shockingly good for such a belated follow-up, and its critical and commercial success convinced the studio to hand the reins to star Anthony Perkins for Psycho 3. This 1986 sequel is a true oddity, playing like a blackly comic art film, with slight touches of Giallo slashers. The movie failed to receive either the good reviews that greeted the previous sequel or reach the same box office. Perkins' would return to the Norman Bates role one last time for 1990's Psycho IV: The Beginning, a TV movie sequel that featured Norman recounting his origin story, with Henry Thomas (The Haunting Of Hill House) playing young Norman in flashbacks.
Psycho IV is well-regarded, but it wasn't the first version that was mooted. During his commentary track on Psycho 3, screenwriter Charles Edward Pogue (The Fly) revealed that he and Perkins were approached by the studio about developing a fourth movie before part 3 had been released. The two developed an outline that would feature some meta, Scream-like qualities, and it opened with Norman back in the asylum following the events of Psycho 3.
This unmade Psycho 4 saw Norman involved in some therapy where a follow, mute female patient would roleplay as his mother. This treatment would involve Norman being put into showers and other sexual situations with "Mother." When the asylum burns down, Norman and the girl escape and head back to the motel, which has been taken over by an entrepreneur who turned it into a murder mystery attraction based on Norman Bates' crimes. As Norman arrives, the actor who was supposed to play him quits, so Bates himself is hired for the part.
Having Norman Bates play himself is already a neat meta touch, but this Psycho 4 would also have been a mystery, as it wasn't clear who the killer was as guests were being bumped off. It sounds like a premise with some potential, but the modest box-office returns of 3 killed that version of Psycho 4 being produced. If it had, it might have predated the meta-textual likes of New Nightmare and Scream by many years, in addition to diving even deeper into the black comedy tone established by Psycho 3.