Tanuga IV is the planet orbited by the research station where Nel Apgar died, and whose laws presumed Riker guilty.
Tanuga IV is the home of the Tanugans, whose legal system treats a suspect as guilty until proven innocent. In A Matter of Perspective, Riker and La Forge visited a research station in orbit of the planet where Dr. Nel Apgar was developing Krieger waves.
The station's destruction led the Tanugan investigator Krag to accuse Riker of murder and seek his extradition. Picard held a hearing aboard the Enterprise instead, and the holodeck reconstructions eventually showed Apgar had died by his own hand, freeing Riker.