I had finished work at about 4 o'clock and then gone to the post office.

因为前边的句子是The time was a quarter past five in the afternoon.And I was driving along the
road to fetch my daughter from her school.Our plan had been to go swimming
together.所以 后面的句子 用过去完成时 表示是在 a quarter past five以前 发生过的事 I可以跟时间点 at about 4 o’clock连用 i had finished work at about 4 o’clock and then gone to the post
office.那现在完成时为什么不能加时间点？因为本句 at about 4 o’clock=before a quarter past five in the afternoon所以 本句可以用 过去完成时 如果 没有前句完成时 是不能和 时间点连用的。前文：The time was a quarter past five in the afternoon. And I was driving along the road to fetch my daughter from her school. Our plan had been to go swimming together.第二句指当时发生的动作 第三句是 第二句动作发生之前的打算 故用过去完成时 表示过去的过去