Zeta Reticuli Four was a planet.
In 1957, the Master interrupted the first Soviet satellite launch and sent a distress signal to the Tzun Canton on Zeta Reticuli Four. He offered to help assimilate Earth into the Tzun Confederacy. In return, the Master asked for passage off Earth and the use of the Tzun's genetic engineering to cure his Cheetah virus infection. The Tzun accepted and prepared nanites for him that broke down the corrupted Trakenite DNA in his cells and restructured it. This was meant to restore the Master to being a "full" Time Lord, which giving him a new regenerative cycle and curing the Cheetah virus. While assisting the Tzun, the Master used the alias "Major Kreer". Shortly after being restored to his full Time Lord heritage he was shot in the back by Ace, causing him to regenerate.
Following the regeneration he was able to make his escape, summoning his TARDIS using a Stattenheim remote control built from Tzun technology. After leaving a booby-trap for the Doctor in a nuclear warhead, the Master fled (PROSE: First Frontier) to the Scoundrels Club to recover from his regeneration. (PROSE: Dismemberment)