In a sense, the Fourth Domain could be considered a mini upper realm. In ancient times, the peak experts of the lower realms would not trigger the Heavenly Tribulation when they cultivated to a certain level. Instead, they would qualify to ascend to the Fourth Domain.
In other words, every living being in the Fourth Domain could be considered an immortal being. The Silver Star Holy Lord, High Monk, Heaven Ascension Sect Master and Dark North Divine Buddha were the cream of the crop among them. They were the overlords of the Fourth Domain.
Ye Qingtang did not believe in Immortal Buddhas back then. But now that she witnessed so much, she had no choice but to believe it.
In Ye Qingtang's era, Immortal Buddhas were just myths and legends. The real existences were True Gods and Heavenly Demons.