The show centers on FBI special agents who work on cases linked to the paranormal, called X-Files; this season focuses on the investigations of John Doggett (Robert Patrick), Monica Reyes (Annabeth Gish), and Dana Scully (Gillian Anderson).
In this episode, a vicious murderer named Erwin Lukesh, capable of jumping between parallel universes, shoots Doggett using Reyes' gun.
Maeda wrote the episode after being inspired by the situation of a real-life French journalist, Jean-Dominique Bauby, who had been paralyzed and could only communicate by blinking his left eyelid.
The character of Lukesh was in part inspired by Alfred Hitchcock's Psycho and was created by Maeda in the hopes that he would go on to be a memorable villain.
Effects expert Mat Beck created the unique visual sequence that showed Doggett and Lukesh traveling between the different universes.
Monica Reyes (Annabeth Gish) monitors Erwin Lukesh, a suspected serial killer who cuts out his victims' tongues.
Reyes then receives a phone call from Walter Skinner (Mitch Pileggi), who informs her that Doggett has been shot and is being taken to the hospital.
As he prepares to fix her lunch, he goes to his freezer and pulls out a bag containing a human tongue, intending to surreptitiously feeding it to her, as he has many times before.
Reyes, stunned and fighting back tears, embraces Doggett as he, evidently unaware of any of the previous events, asks her what is wrong.
Maeda was inspired to write the episode after learning the story of a French journalist named Jean-Dominique Bauby, who had been paralyzed and could only communicate by blinking his left eyelid.
Maeda later noted that he wished to create a villain that would be as memorable as other antagonists in The X-Files, such as Eugene Victor Tooms and The Flukeman.
He noted, "I'd read The Diving Bell and the Butterfly, and I believe there's a part of that in the book where the character gets shaved, and I thought, 'Wow, that's really an intimate kind of moment.
'"[4] Effects expert Mat Beck was instructed to create a visual sequence that showed Doggett and Lukesh traveling between the different universes.
[3] Various effects were tested, including characters disappearing into black holes as well as phasing through the air in a more "exaggerated" and "watery" way.
[3] To create the effect of a parallel universe, every scene in the episode's teaser was flipped so that the mirror image was shown.
Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated the episode four stars out of five.
"[11] In a season review, Michelle Kung from Entertainment Weekly wrote that "4-D" was one of the few ninth-season episodes "worthy" of praise, but that it was overshadowed by the show's "ludicrous conspiracy plots".
Crang, in his book Denying the Truth: Revisiting The X-Files after 9/11, felt that the interactions between Robert Patrick and Annabeth Gish were the "highlights" of the episode, but he was critical of the ending for wrapping things up "far too conveniently.