The Wikipedia source is given as "Reu (1933), p39". This refers to History Of The Rashtrakutas by Pandit Bisheshwar Nath. Reu. According to this text, which can be found on the Internet Archive, the Silsilatuttavarikh is a work by an Arabic trader written in 851. Reu quotes the book as saying:

The inhabitants of India and China agree that there are four great or principal kings in the world. They place the king of the Arabs (Khalif of Baghdad) at the head of these........The king of China reckons himself next after the king of the Arabs. After him comes the king of the Greeks, and lastly the Balhara, prince of the men who have their ears pierced (i.e., the Hindus).

The Balhara refers to the Rashtrakutas, so the other three empires would be the Abbasid Caliphate, China, and Rome (Byzantium).