Balāš IV reigned from ca. 147/8 to 190/1 or 192/3. In the first part of this king’s reign, Roman-Parthian relations remained peaceful. Balāš IV, however, was probably waiting for a chance to settle the Armenian problem on terms more favorable to Parthia. When the imperial throne passed to Marcus Aurelius in 161, Balāš saw the time as ripe. For the first (and last) time, the Parthians declared war on the Romans. No precise information on the reasons for this step has come down. Ziegler (p. 112) may perhaps be right in his surmise that Balāš IV had long been unwilling to drop the Parthian claim to influence in Armenia. Initial successes appeared to justify Balāš’s calculation. A Roman army was defeated at Elegeia on the Euphrates in 161, and Armenia was occupied and placed by Balāš under a new king named Pacorus (Chaumont, pp. 147-48). At the same time, the Parthians invaded Syria. The Romans, however, despite their initially precarious situation, soon began to counterattack. Lucius Verus, then co-emperor with Marcus Aurelius, took over their supreme command and had the support of the able generals C. Avidius Cassius, M. Statius Priscus, and P. Martius Verus. In 163 they overran Armenia and installed a new king, Sohaemus (Chaumont, pp. 149-50). After expelling the Parthians from Syria, they began to penetrate into Mesopotamia in 164/5. Dura Europos fell to them after hard fighting and thenceforth remained in Roman hands. At this juncture many of the Parthian vassals and their troops deserted Balāš (again showing how much the Parthian great kings depended on the local rulers within the empire: see Wolski, pp. 379ff., 385-86). As a result, Seleucia was captured by the Romans some time after December, 165. Whether the city was then really destroyed, as scholars relying on ancient authors often assert, seems doubtful because coins were again being minted at Seleucia in November, 166 (Hopkins, p. 161; Debevoise, p. 251, n. 58). The nearby royal capital, Ctesiphon, was also taken, and Balāš IV’s palace was demolished. Only the outbreak of a plague saved the Parthians from crushing defeat and forced the Romans to withdraw. It is not clear from the sources whether the two sides concluded a formal peace treaty (Ziegler, p. 114, and Schur, col. 2025, assume that they did, while Debevoise, pp. 252-53 says nothing to the contrary). Rome’s new eastern frontier ran from Dura Europos northward along the Ḵābūr river, and the region west of this line, including not only Edessa but also Carrhae and Nisibis, belonged to the Roman empire. (The exact line is uncertain, and there is disagreement on the question whether these cities and Singara were annexed in the reign of Marcus Aurelius or not until that of Septimius Severus. See Ziegler, p. 114, n. 131; Magie, pp. 15, 44; Oates, p, 72; Bertinelli, pp. 41ff.)

In the following decades peace between the two empires prevailed. An opportunity for the Parthians to cause trouble for the Romans on their eastern frontier arose early in the year 175 when Avidius Cassius, the victor of Ctesiphon, proclaimed himself emperor in Syria. It is not known whether Balāš contemplated intervention in the prospective civil war between this pretender and Marcus Aurelius, but if so, he failed to act in time, because Avidius Cassius was put to death by his own troops a few months later. Soon afterward Marcus Aurelius traveled to the East and confirmed the state of peace between Rome and Parthia in talks with Balāš IV’s envoys at Antioch in the summer of 176. The peace endured throughout the reign of Marcus Aurelius and his son and successor Commodus (180-92).

Balāš IV died in 190/1 or 192/3. (Le Rider, p. 461, and Sellwood, p. 267, give the earlier date, but McDowell, p. 235, and Hanslik, col. 1851, state that coins of Balāš IV were minted as late as 193, and Debevoise, p. 255, accepts the later date.) Since coins of Balāš V dated 191 also exist, it is possible that Balāš IV had made his son co-regent (surmise of Hanslik, loc. cit.) or that a civil war between the father and the son had broken out (surmise of Debevoise, loc. cit.). There are also some coins from ca. 190 minted for a king who (according to Sellwood, p. 281) bore the name Osroes (previously often misread as Artabanos); apart from the coins, nothing about him is known.

Balāš IV managed to hold the Parthian throne for more than forty years. He suffered defeats at the hands of the Romans but did not have to deal with the usual internal troubles due to revolts of pretenders and resultant civil wars (unless the surmise of a civil war with his son Balāš V is correct).