Re: ????3wt or 4wt line????
One way to look at those ratings is to use the lower number if you get a DT line and the higher if you get a WF line. I made my wife a rod rated 4/5 and picked up an Orvis Clearwater 4wt line that is weighted 1/2 line wt higher and it casts very nicely.
The best would be to have a buddy with reels with 3wt and 4wt line to try, but if it were me I'd probably just get a DT4wt line. I like DTs and with lighter rods I tend to fish in closer so like the extra weight to load the rod with less line out the tip.