As a dominion, Sirius IV had great autonomy within the Empire; it dealt with Earth mostly as an equal. The Master once used this special status as a way to negotiate for the release of the Third Doctor and Jo Grant into his custody. By fabricating charges against them on Sirius IV, he was able to successfully argue that he had greater claim to the prisoners than Earth Security.
Behind the scenes Edit
According to the novelisation the planet was called "Alderberan Four", and it had only recently become a dominion.