Marge vs. the Monorail - S4-E12
Trivia: Conan O'Brien, who wrote this episode, did a live rendition of "The Monorail Song" at the Hollywood Bowl in 2014 with the Hollywood Gay Men's Choir and Hank Azaria. He dedicated the performance to Phil Hartman.
Krusty Gets Kancelled - S4-E22
Trivia: When this episode aired in Mexico, the name of Luke Perry was translated as - believe it or not - Robert Redford... This was because even though Luke Perry was very popular in that country thanks to 'Beverly Hills 90210', the translator did not know him, and absurdly and stupidly assumed no one else in the country did either, so the name of another 'more popular' celebrity was used.
Kamp Krusty - S4-E1
Trivia: At the end of the show, the person singing 'South of the Border' is actually creator Matt Groening. Confirmed on DVD Commentary.
Trivia: You can see Matt Groening laughing at Itchy & Scratchy in the audience of the Annual Cartoon Awards.
Marge vs. the Monorail - S4-E12
Trivia: The entire premise of this episode, where a man in turn-of-the-century garb sings and charms his way into the townspeople's hearts as part of an elaborate con, is a takeoff of the musical, "The Music Man" (1962). Even the tunes of the songs they sing are similar.
Itchy and Scratchy: The Movie - S4-E6
Trivia: Lisa refers to Dustin Hoffman and Michael Jackson voicing guest characters in the "Itchy & Scratchy" movie. She says "They didn't use their real names, but you could tell it was them." This is a sly reference to Hoffman and Jackson's pseudonymous appearances in The Simpsons, in Seasons 2 and 4, respectively.
Last Exit to Springfield - S4-E17
Trivia: Mr Burns' reaction - right down to his animation - to the singing of the Union towards the end is a direct spoof of the Grinch (from The Grinch Who Stole Christmas) by Dr. Seuss. His characters also spoke in rhyme, which is why some of Mr Burns' lines are nonsense.
I Love Lisa - S4-E15
Trivia: Ralph Wiggum was named after Ralph Kramden on "The Honeymooners" because the character was intended to be a loudmouthed smaller version of Homer. He wasn't established as Chief Wiggum's son until "I Love Lisa."
Trivia: Lisa says she is laughing at a joke from "Herman's Head." Yeardley Smith, the voice of Lisa, was one of the stars of "Herman's Head" (1991).
Trivia: Marge is accused of shoplifting and hires Lionel Hutz, played by Phil Hartman, as her attorney. In the middle of the trial, Lionel gets an urge for whisky, so he calls his AA sponsor, David Crosby of Crosby, Stills, Nash and Young. When Crosby answers the phone he is looking at the CSNY emblem on an album, which Phil Hartman designed himself in the late '70s.
Trivia: The line "If you should die before you wake," uttered by the clown bed is a copy of a scene from the film 'A Nightmare on Elm Street'.
Kamp Krusty - S4-E1
Trivia: James L. Brooks, the executive producer, was so enthusiastic about the episode's premise that he wanted to make it the official Simpsons movie. This idea was abandoned because the writers had problems getting the story to 23 minutes, let alone 90 minutes.
Trivia: When Homer sees the Frying Dutchman ad, he blisses and lean backwards. This is exactly the same shot from "Bart's Friend Falls in Love", when Homer is watching a program about chocolate on TV, only the frame is flipped. (00:04:30)
Krusty Gets Kancelled - S4-E22
Trivia: The scene where Bette Midler sings "Wind Beneath My Wings" to Krusty is an allusion to Johnny Carson's penultimate episode as host of The Tonight Show where Midler was his final guest and sang to him. She even called Carson the wind beneath her wings.
Answer: Willy referred to an encounter he believed he had with actual aliens, while Wiggum thought he was referring to the arcade game.