It that 강조구문에서 that의 역할이 궁금해요

It that 강조구문에서 that이 어떤 역할을 하는지 궁금합니다.
“He met her in front of the bus stop yesterday.”를 아래처럼 강조할 수 있다고 배웠어요.

It was he that met her in front of the bus stop yesterday.
It was her that he met in front of the bus stop yesterday.
It was in front of the bus stop that he met her yesterday.
It was yesterday that he met her in front of the bus stop.

네 문장의 that을 모두 관계대명사로 보면 되나요?
뒤 문장이 완전한 세 번째, 네 번째 문장에서는 접속사나 관계부사로 보는 건지 궁금합니다.