4K Box

Went to my local Xfinity Store (Mercer Mall, Lawrenceville) to swap my 4K box for a new one, as my existing box needed to be power cycled every day to get it working.  At the store, I was told they did not have any more boxes like mine and was given a different style box, which I was told was a 4K box, just a different brand.  After hooking up the new box, it is clear to me that it is an HD box, not a 4K box and the representative at the store lied to my face, as I'm sure they would have known the difference.  I don't see the point in having a 4K TV without a 4K box.  As I was told they don't have any 4K boxes, can this box be swapped via mail?