Delta IV (or simply Delta) was the inhabited fourth planet in the Delta system, in the Alpha Quadrant, and was the homeworld of the humanoid Deltans. The planet was a Federation member world. (Star Trek: The Motion Picture; Star Trek IV: The Voyage Home)
Several years before his assignment to the USS Enterprise in the 2270s, Will Decker met Ilia on Delta IV, where they became romantically involved. After V'Ger had replaced her with a near-duplicate, Christine Chapel tried to activate her original memories with a head dress she used to wear back on Delta. (Star Trek: The Motion Picture)
In the writers/directors guide for Star Trek: Phase II, the Deltan homeworld was referred to as 114-Delta V. The same document also stated that the planet had "recently joined the Federation." (Star Trek Phase II: The Lost Series, p. 90) In the first-draft script for "In Thy Image" (the story that gradually transformed into The Motion Picture – the script was dated 20 October 1977), the Deltan homeworld was referred to as Delta 14. (Star Trek Phase II: The Lost Series, pp. 121 & 139)
In November 1977, consultant company de Forest Research addressed the naming of the Deltan homeworld, stating that the word "Delta" being used as a planet name presents a problem in astronomy, since it (in common with other Greek letters) is commonly used to identify a particular star in a generic, recognized constellation. "That it would ever be used as a planetary name is very unlikely," the research company declared. They concluded, "We would be happy to consult on other possibilities which might be more in accord with standard astronomical usage." (Star Trek Phase II: The Lost Series, p. 64)
Evidently, the production crew did not adhere to the advice from De Forest Research, and variations of the name were included in several examples of documentation that were much more closely related to The Motion Picture than they were to Phase II. In a character study of Ilia that Gene Roddenberry wrote for the film, the Deltan homeworld was referred to as Delta. (The Making of Star Trek: The Motion Picture, p. 108) The planet was also mentioned in notes that costume designer Robert Fletcher wrote, regarding the various aliens in the movie. In that case, the Deltan homeworld was called "Planet Delta 4." (The Making of Star Trek: The Motion Picture, p. 133)
According to Star Trek: Star Charts (p. 33; "United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the M-class planet Delta IV was governed by the Delta Union (β). This world joined the Federation in 2223, the year the Deltans became warp-capable. In the 2370s, there were 3.8 billion Deltans living on the planet. Instead of exposing offworld humanoids to Deltan pheromones, they were directed to the moons of Seyann (β) and Cinera (β). This planet was located in the Delta system.
According to The Worlds of the Federation (p. 64), Delta was the fifth planet orbiting Delta Triciatu (β), a white dwarf star. The indigenous name for the planet Delta was Seyalia. The planet was covered by vast forests and had two moons, Seyann and Cinera.
According to Who's Who in Star Trek 1, Delta was the fifth planet in the 114-Delta system.
In Articles of the Federation, Delta IV is mentioned to be attacked by the Dominion during the Dominion War. This resulted in the contamination of their water table, and by 2379, the breakdown of their water reclamation system. Delta attempted to obtain a new Carreon-designed system, and after four months of negotiation, agreed to allow a Carreon scientific base on the planet Brannik IV in exchange for this new system.
In Star Trek: The Next Generation - Doctor Who: Assimilation², Delta IV is attacked by the combined force of the Borg and Cybermen as their first attack against the Federation on Stardate 45635.2. The largest cities were targeted, followed by the destruction of command and control infrastructure. The USS Potemkin and the USS Lassiter (β) respond to the attack, but a Borg cube immediately destroys the Lassiter and heavily damages the Potemkin. At least half of the planet was overwhelmed in a matter of minutes, forcing its prime minister to order a planetary evacuation well before Starfleet's defensive task force could arrive. The USS Enterprise-D later responded to the distress call and arrived in the Delta system, but was immediately fired upon by the Borg/Cyberman fleet and forced to flee while the Borg–Cyberman fleet give chase. In the end, the work of the Enterprise crew and the Eleventh Doctor and his companions led to the prevention of the Borg/Cyberman alliance, so the events at Delta IV were erased from history beyond the people involved maintaining their memories of the events.