I have the Tioga 6 on my St. Croix Imperial 5wt. Balance is good and there are no problems casting ( except for my limited skill! ). The #6 is only slightly larger and heavier than the #4. It might be better to go with the #6 in case you want to go up a weight or two in the future. All you'd need is an extra spool.

Regards

P.S. Good choice on the Tioga!