Re: 8.5 or 9 ft for a 4wt?
What rod (brand / model) are you considering?
I have 4wts ranging from 7'6" to 9'6". I find no difference at with the extra 6" in the same rod brand / model. It will surprise some about how long of a rod you can use on small streams. Don't go by the popular adage that one needs a super short rod for small streams. For e.g. on majority the mountain streams in Appalachia, I comfortably fish my 8'6" 4wt since there is usually enough slot behind me as long as I am observant of the room behind me. The longer rod length permits minimizing drag (keep most of fly line off water) and also I can reach the pocket water more effectively while staying hidden.
Depending on the rod action, the extra 6" may chance the characteristics when Nymphing since it allows slightly easier mending & some minor advantage in roll casting...but that is largely a function of the rod action.