Minamoto no Yoshimune (源義宗)
He died young, but it is unknown whether he died in battle or of illness. He was appointed as Hyogo no jo (an officer of Bureau of Military Storehouses) and Sa-emon-shoi (Jr. Lieutenant of the Left Division of Outer Palace Guards) ("Sonpibunmyaku" [a text compiled in the 14th century that records the lineages of the aristocracy]). He also adopted his nephew, MINAMOTO no Tadamune (the third son of MINAMOTO no Yoshitada).
In the first volume of "Hiraizumishi" (The History of Hiraizumi) (edited by Mafuji TAKAHIRA, the head of the Koseikan runned by the Ichinoseki Domain), there is an article about the battle at Kanazawasaku in Go Sannen no Eki (Later Three-Year Campaign):
Yoshiie-Ason (title) could not go to the front because of tiredness.'
His younger brother, Shinra-SaburoYoshimitsu, and his legitimate child, Yoshitada, who was Kawachi hangan (judge, magistrate), led the army and attacked Kanazawa.'
This article describes the battle in 1090, but MINAMOTO no Yoshitada was only seven years old at the time, so it is impossible to think Yoshitada led the army.
Therefore, if the person who led the army is thought to be one of the other brothers and the legitimate child is taken as the word that means just the first son, the legitimate child of the above article is thought to mean Yoshimune.