How is Hippolyta's reasoning concerning how quickly the next four days will pass different from that of Theseus?
Hippolyta's vision of the four days is probably quite different from Theseus's. As you probably know, Theseus has brought Hippolyta back to Athens as a prize from his successful war against the Amazons. This might make her a less than enthusiastic bride. Theseus, looking forward to the wedding, worries that time will pass slowly, while Hippolyta, perhaps not so eager for the wedding, knows that the time will pass quickly.
There is not a great deal in the text to support this however. It can be read between the lines - in Hippolyta's relative silence.