Dave is correct. You can find a 4X but will have to search classifieds I think to find a 6X, in fact I don't recall ever seeing a 6X Nikon scope.
Best of luck, if I can help you with any Nikon product's drop me a note.
You could choose a 3-9,2-8, 1.5-6 with 30 mm tube?
Nikon Pro Staff