Title: Mithridates IV of Parthia
Content:  Mithridates IV (also spelled Mithradates IV; Mihrdāt) was a Parthian king from to 57 to 54 BC. He was the son and successor of Phraates III ((r. 69 – 57)). Mithridates IV's reign was marked by a dynastic struggle with his younger brother, Orodes II, who eventually emerged victorious and had Mithridates IV executed, thus succeeding him.
Title: Mithridates I of Parthia
Content:  Mithridates was the son of Priapatius, the great-nephew of the first Arsacid king, Arsaces I ((r. 247 – 217)). Mithridates had several brothers, including Artabanus and his older brother Phraates I, the latter succeeding their father in 176 BC as the Parthian king. According to Parthian custom, the reigning ruler had to be succeeded by his own son. However, Phraates I broke tradition and appointed his own brother Mithridates as his successor. According to the 2nd-century Roman historian Justin, Phraates I had made his decision after noticing Mithridates' remarkable competence.
Title: Mithridates II of Parthia
Content:  since Mithradates II's natural contestants for the throne would have been the sons of Phraates II and Mithridates I." Unlike Mithridates I and Artabanus I (who were the sons of Priapatius), Mithridates II did not use the title of Theopatoros ("whose father is a god"). This brother named Artabanus is not referred to as king in cuneiform, and was most likely a high-ranking officer who died in war. The identity of Mithridates II's father is uncertain. According to 2nd-century Roman historian Justin, Mithridates II was a son of his predecessor, Artabanus I. A cuneiform tablet from 119 BC, however, cites Mithridates II; "of the Gutians who killed my brother Artabanus, and I set up (troops) opposite them and fought it with them; a great killing I performed among them; except two men." 
Title: Mithridates I of Parthia
Content:  Mithridates I (also spelled Mithradates I or Mihrdad I; Mihrdāt), also known as Mithridates I the Great, was king of the Parthian Empire from 165 BC to 132 BC. During his reign, Parthia was transformed from a small kingdom into a major political power in the Ancient East as a result of his conquests. He first conquered Aria, Margiana and western Bactria from the Greco-Bactrians sometime in 163–155 BC, and then waged war with the Seleucid Empire, conquering Media and Atropatene in 148/7 BC. In 141 BC, he conquered Babylonia and held an official investiture ceremony in Seleucia. The kingdoms of Elymais and Characene shortly afterwards became Parthian vassals. In c. 140 
Title: 171 BC
Content: Mithradates I succeeds his brother Phraates I as king of Parthia. 
Title: Mithridates II of Parthia
Content: Son of Priapatius: According to Vesta Sarkhosh Curtis (2019), Mithridates II was referring to Artabanus I as his brother in the cuneiform. She argues that Mithridates II was a son of Priapatius (also spelled Friapatak), which is also supported by an ostracon created in 91/90 BC in Nisa. On the ostracon, a Parthian king, most likely Mithridates II's son Gotarzes, is mentioned as "Arsaces King, grandson of Friapatak [who is] the son of the nephew of Arsaces [I]." ; Son of Artabanus I: Olbrycht (2010), however, suggests that Mithridates II was not Artabanus I's brother, but a son of his. He states that Mithridates II would have been a middle aged man the time of his accession, due to Priapatius dying in 176 BC. According to Olbrycht, "While not impossible, it is historically 
Title: Mithridates V of Parthia
Content:  Mithridates V ( Mihrdāt) was a Parthian contender from 129 to 140. His son, Vologases IV of Parthia (147&ndash;191), took the throne after the death of Vologases III in 147.
Title: Mithridates II of Parthia
Content:  Mithridates II (also spelled Mithradates II or Mihrdad II; Mihrdāt) was king of the Parthian Empire from 124 to 91 BC. Considered one of the greatest of his dynasty to ever rule, he was known as Mithridates the Great in antiquity. Mithridates II was crowned king after the abrupt death of his predecessor Artabanus I. Inheriting a declining empire that was reeling from military pressure in both the east and west, Mithridates II quickly stabilized the situation in Mesopotamia by gaining the allegiance of Characene, and subduing the insurgent Kingdom of Elymais and also the Arabs, who had continuously raided Babylonia. Mithridates II 
Title: Mithridates (Persian general)
Content:  Mithridates or Mithradates (Μιθριδάτης or Μιθραδάτης) was a Persian noble. His wife was the daughter of Darius III with the sister of Pharnaces, which makes him son-in-law of Darius. He was slain by the hand of Alexander the Great himself, at the Battle of the Granicus in 334 BC, when Alexander plunged his lance through Mithridates' face.
Title: Mithridates IV of Parthia
Content:  Mithridates IV was a son of Phraates III ((r. 69 – 57)), under whom he served as the ruler of the central province of Media. In 57 BC, Mithridates murdered his father with the assistance of his younger brother Orodes. However, the two brothers quickly fell out, and Orodes revolted with the support of the Suren clan. They both assumed the title of King of Kings to demonstrate their claims of superiority over each other. This changed the meaning of the title; originally being used as a symbol of political dominance over other realms, the title became known as a symbol of power and legitimacy for contenders in a royal family. Mithridates IV was forced to flee from to Roman 

Question: Who is the father of Mithridates I of Parthia?
Answer: Phriapatius of Parthia.
Phriapatius.