A new click wheel is a cheap way to start and it sounds like that is all you need from your description. The issue is it is a Nano 4th! The issue with the 4th and 5th gen's is the tolerances are so tight and the ribbon cables are so fragile and small they tend to get torn off. Go very very slow and careful! Follow the guide here:
iPod Nano 4th Generation Click Wheel Replacement
I can't express it enough so I'll say it again - very very slow and careful! If you haven't done repairs on these before I would suggest you take it to a professional.