After the death of his co-ruler Pacorus, the Parthian king Orodes II appointed his son Phraates IV as successor. The crown prince did not wait for his father to die but had him killed, together with some of his brothers (reportedly no less than thirty). He was now uncontested sole ruler of a kingdom that was at war with the Roman Republic, which still had some accounts to settle after its defeat at Carrhae (in 53 BCE) and Pacorus' war in Syria (41-38 BCE).
In 34 and 33 BCE, the Roman commander Mark Antony attacked the Parthians in a war that is presented in the Roman sources (which are hostile toward Mark Antony) as a calamity. The reality may have been less disastrous, because the Parthians revolted against Phraates and made him flee. Their new king was Tiridates I, but Phraates returned with support from the Scythians.
It was now obvious to the Parthians that the Romans could be dangerous and from now on, they refrained from attacks across the Euphrates and agreed to find a solution for a potential diplomatic problem: the succession of Artaxiad kings in Armenia. In 20 BCE, after negotiations conducted by Tiberius, the Roman emperor Augustus and Phraates decided that Armenia was to be independent but that the Romans had the right to appoint and crown its kings. This treaty was to be in force for more than seventy years.
Soon afterwards, Phraates married to an Italian lady, named Musa, who is the victim of all invectives the ancient storytellers knew: she had been a slave, she committed incest, was responsible for the death of her husband.note[Josephus, Jewish Antiquities 18.40-43.] He also sent his sons Seraspadanes, Rhodaspes, Phraates, and Vonones to Romenote[Augustus, Res Gestae 32; Strabo, Geography 16.1.28; Josephus, Jewish Antiquities 18.42.] - an investment in better relations and a guarantee that they would not be killed in a palace coup.
A coup appears to have been inevitable indeed. Shortly after he had appointed a son of Musa, Phraates (nicknamed Phraataces, "little Phraates"), as successor, Phraates IV was killed.note[Josephus, Jewish Antiquities 18.39 and 18.42.] (Phraataces would be succeeded by his brother Vonones I, a son of Phraates IV who had been living in Rome.)
Phraates IV is credited with building projects in Ctesiphon, a new capital on the Tigris.
The chronology of the Arsacid kings of the Parthian Empire is less well-understood than, for example, the sequence of Seleucid and Ptolemaic kings or the emperors of Rome. This information is based on the researches by G.R.F. Assar, as published in "Iran under the Arsakids, 247 BC – AD 224/227" in: Numismatic Art of Persia (2011).