前面用介词at，例如：

1.I play games after school with my friends and then go home at half past four.放学后我和我的朋友们玩游戏，然后在四点半回家。

2.The children played for an hour, and then at half past four their aunt took Dick into the kitchen.孩子们玩了个把小时，在四点半的时候，姨妈领着迪克走进了厨房。

四点半是个具体的时刻，前面介词用at