Must I make up the prayers I performed while in a state of major ritual impurity (junub), knowing that I did not perform ghusl due to the unavailability of hot water at the time, and approximately four years have passed since then?

Wudu (ablution) is not sufficient for ghusl (ritual bath) or tayammum (dry ablution). Therefore, the Fajr prayer and all subsequent prayers performed while in a state of janabah (major ritual impurity) without ghusl or tayammum must be re-offered. If the exact number is unknown, the prayers must be re-offered until one's conscience is clear.