Just a complete guess here, but I would GUESS you wanted the 5.1ounce reel, since they say that is for line weights 5-6 and you would (I'm assuming here) match the line weight (5 weight line) with the rod rating (5 wt rod). If you're planning on under-lining the rod with a 4wt line, then you can go with the 4.1oz reel, but I doubt that's what you'd want.
I don't know if the Bass Pro would let you, but maybe you could take your rod in and try to cast with a lined reel from them, or at least you could feel the weight of it and balance on the rod. Just a thought... If not, see what their return policy is in case it isn't to your liking...
Hope that it helps, and by all means, ask questions - we're here to help!
-ZugbugPete