In the schoolhouse, Michael's mask appears to have long, blonde hair. This is because the mask is a second type of mask made for the film and intended to use it, however the mask with darker hair was chosen but during re-shoots, the blonde mask was used and seen in multiple parts of the film, although the school scene is the only replacement mask.
In the original script, the film opened with a shot of a long hospital corridor suddenly blowing up and throwing Loomis from the explosion, in a reference to the end of 'Halloween II (1981)_ ' in order to show how Loomis survived. It was later decided the film should not have any connections to the predecessors and the explosive opening was never shot.
In late 1986, The Cannon Group (producers Menahem Golan and Yoram Globus) contracted producer Debra Hill and director John Carpenter to work on a screenplay for a new Halloween - to be titled Halloween IV - as they were interested in optioning the rights, much like the studio had for Texas Chainsaw Massacre 2. Cannon did not like the treatment that Carpenter and Hill turned in and ultimately decided not to produce the film. Shortly afterwards, Carpenter and Hill sold all their remaining interest in the Halloween series to producer Moustapha Akkad - who promptly developed Halloween 4: The Return of Michael Myers - with a completely new script, fashioned around the slasher film boom of the 1980s.
After viewing a rough edit it was decided that the movie was too soft, so they brought in special effects wizard John Carl Buechler for one day of extra "blood" filming. The thumb in the forehead and the redneck's head getting twisted were both done by him.
The gaffer, Garlan Wilde, was seriously injured during the filming of the Michael and Brady confrontation. Garlan was putting up a light and fell and cut his wrists; he was quickly rushed to the hospital.
During production of the rooftop chase, Ellie Cornell was injured by a protruding nail as she slid down the roof. After a quick trip to the local hospital she finished the scene with her bandages in place. According to Danielle Harris, "It didn't even faze her."
In the original script, Sheriff Meeker was killed in a battle with Michael in the basement where the furnace was knocked over and caused the house to catch on fire. Originally, the house was supposed to be up in flames during the infamous rooftop sequence. This was eliminated due to budgetary constraints and Sheriff Meeker was kept alive.
Series creator John Carpenter wrote a treatment for this film, that was a more ghostly psychological approach to the Michael Myers mythos. It concerned the town of Haddonfield and what effect the events of the first two films have had on its citizens. This concept was later rejected by the producers in favor of the typical slasher fare, at which point in time John Carpernter bailed out of the film, making this the first film in the series to have no participation from him.
Originally, when Jamie and Loomis were trapped in the school, Jamie hid in a classroom under a desk. Michael entered searching for her, throwing the desks over. Although they had no time to film this in Halloween 4, the sequence was remembered by Moustapha Akkad and later re-used in Halloween H20: 20 Years Later (1998).
In Jamie's introduction, she's sitting in the living room staring outside at the ambulance. Later, it shows the ambulance has disappeared. In the script, Jamie was staring outside at the rain, and the ambulance appeared after she had turned away. This was changed in editing for unknown reasons.
George P. Wilbur wore hockey pads under the jumpsuit to give Michael Myers a much more imposing figure. This is revealed in the documentary "Inside Halloween 5", where it is revealed that Don Shanks, who played Michael Myers in Halloween 5 (1989), was big enough that this was not required.
The girl Lindsey who drove Rachel and Jamie to the discount mart was going to be revealed as Lindsey Wallace from Halloween 1 and have a larger part in the film as Rachel's friend and helping her babysit Jamie, but was cut for budget reasons
Like the doors of the house where Laurie Strode found the bodies in Halloween (1978) , the doors and windows of Sheriff Meeker's house have locks that need keys to lock/unlock them from their insides, causing hazards to anyone needing to escape from danger if the keys are lost.
The trivia item below may give away important plot points.
The original ending had Rachel go upstairs to take a shower and replace the dirty, blood-stained clothes she was wearing only to have Jamie sneak up behind her and stab her to death. This scene was later changed to Darlene Caruthers but this scene was remembered for H5 and was used for Rachel's death scene.