It was four o'clock in the afternoon ( ) he and his team reached the top of the mountain.

强调句特点是去掉it is 和that 依旧成立

这里
he and his team reached the top of the mountain (at) four o'clock in the afternoon
少一个at不成立，是先行词作状语的定于从句，不是状语从句

少介词. It was at four o'clock ...