Question

Hi, I am learning English.

Answer 1

Hadn't been, is the same as "had not been"

inside the sentence, It had not been until the Fourth Dynasty that the most famous pyramids were built, had not been does not fit inside (it doesn't make sense).

Wasn't works, because wasn't is the same as was not

It was not until the Fourth Dynasty that the most famous pyramids were built.

This works as it creates a normal sentence

hope this helps