Doing a quiz called Cash Trove and this is one that I am stuck on...

I'd say 9312.

There are 7 numbers right? Well, if you put a line of symmetry on the middle number, you'll notice that:
7288 - 728 (8)
7178 - 717 (8)
3418 - 341 (8)

Only 9312 doesn't have its pair :)