Vologases, during his early reign, united the two halves of the empire which had been split between his father and Vologases III of Parthia (105–147). He also reconquered the kingdom of Characene which seems to have been independent since the Roman invasion of the Parthian empire under Trajan (98–117). Vologases IV may be the king Volgash of the Zoroastrian tradition, who began the gathering of the writings of Zoroaster.
Conflicts with Rome began in about 155 with a dispute, as usual, over the Kingdom of Armenia. During the reign of the emperor Marcus Aurelius, the Parthians attacked Armenia and deposed Sohaemus, the Roman client king there. Although the Roman–Parthian War (161-66) started auspiciously for the Parthians, after the Romans recovered from the first shock and setbacks, they counterattacked, restored Sohaemus to the Armenian throne and invaded the Parthian Empire.
In this war the city of Seleucia on the Tigris was destroyed and the palace at the capital, Ctesiphon, was burned to the ground by Avidius Cassius in 165. The Roman legions advanced as far as Media. Vologases IV made peace but was forced to cede western Mesopotamia to the Romans.
The end of his reign was marred by the revolt of Osroes II of Parthia (190), who appears to have set himself up in Media as a rival King in hope of succeeding him. In the event, Vologases' son, Vologases V (191–208) of the Arsacid cadet branch dynasty, ruling Armenia, won the succession, and appears to have quickly put down Osroes II.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Meyer, Eduard (1911). "Vologaeses s.v. Vologaeses III. (sic)". In Chisholm, Hugh. Encyclopædia Britannica. 28 (11th ed.). Cambridge University Press. p. 196.
- Parthian Coins, David Sellwood, The Cambridge History of Iran, Vol.3, Ed. E. Yarshater, (Cambridge University Press, 1983), 297.
- Toumanoff, Cyril (1986). "Arsacids". Encyclopaedia Iranica, Vol. II, Fasc. 5. Cyril Toumanoff. pp. 525–546.
- Chaumont, M. L. (1988). "BALĀŠ". Encyclopaedia Iranica, Vol. III, Fasc. 6. pp. 574–580.
- Dio Cassius, lxxi, 1.
- Augustan History, Marcus Aurelius, 8; Verus, 8.
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