In a parallel universe, Delta IV was a Federation planet, located in the Delta system.
- You may be looking for the unrelated moon, Delta III.
The Eleventh Doctor, of N-Space, claimed to have never heard of a Delta IV, despite claiming to know every planet of every star system.
It was the location of the first joint Borg/Cybermen attack against the Federation in 2368. The planet was overwhelmed in minutes, forcing its Prime Minister to order a planetary evacuation well before Starfleet's defensive task force could arrive. (COMIC: Assimilation²)