Why was the era of the Rightly Guided Caliphs given this name?

The Prophet, peace and blessings be upon him, mentioned that prophethood would exist, then Allah would elevate it, then there would be a caliphate upon the prophetic methodology, then a tyrannical kingship, then an oppressive kingship, then a caliphate upon the prophetic methodology would return. It is agreed that the era of the four Caliphs: Abu Bakr, Umar, Uthman, and Ali, was the era of the caliphate upon the prophetic methodology, and they are the Rightly Guided Caliphs whose Sunnah we are commanded to follow.