Four Major Sects are the four strongest profound sects in Blue Wind Empire. Their level of strength is head and shoulders above any other sect so much so that there is no sect that can claim to be the fifth sect. A distant fifth is the Heavenly Spear Thunderfire Fortress.
The process in which they recruit disciples was very strict and they did not care about background in their selections. The most important thing they want in the end was talent. 
One of the main reason why there were only Four Major Sects in the Blue Wind Empire was because these sects possessed practitioners at the Emperor Profound Realm.
Current ranking from strongest to weakest:
- Frozen Cloud Asgard (potential Sacred Ground)
- Heavenly Sword Villa
- Xiao Sect
- Burning Heaven Clan (extinct)
Initial ranking from strongest to weakest:
The idea of Four Major Sects is no longer applicable later on in the story as the Burning Heaven Clan was extinguished and Frozen Cloud Asgard transcended to the realm of the Sacred Grounds.