4.Don't you think 5,500 miles___(is,are)quite a long distance?

is
5,500 miles整体作为第三人称单数看待。

Don't you think 5,500 miles is quite a long distance。

例如：
Twenty miles is a long way to walk.
二十里地可是个不近的路程。

is，主语是个整体概念

is