I will take Ueno than narith city anyday! Ueno has a lot of discount sports store, good toy/model puzzle place, and not to mention the Ueno park( if you are in to walking Japanese style) 4,5 hours is definitely more than enough there and having sushi/ramen lunch.

It is not going to be rush, just remember to book the return ticket once you get off the train, then you know when you need to come back. Also, there are lockers in the Ueno skyliner train station, if you had any hand carry, you can put it there. There is always a locker person who would exchange cash for you into coins. ^