Lighter is better! A few years ago 5 and 6 weight fly rods were common sights on UK rivers. In most cases this is/was certainly a case of using a sledgehammer to crack a walnut. I use a 3 weight fly rod for my dry fly fishing. The model is a Sage Light Line 389 and is 8 feet 9 inches in length. This isn't just my favourite dry fly rod it is my favourite piece of fly fishing equipment period. In my opinion this was one of the best stream rods ever produced. In this clip the advantages of lighter line fishing are explained.
Of course the phenomenal Sage Light Line series was discontinued. However any progressive middle actioned rod around 9 feet in length and rated AFTMA 4 is the most useful all round river rod for dry fly fishing. It has enough power to handle most situations and fish combined with the sensitivity to enjoy the fishing and the 10-12 inch trout found in most streams. River rods smaller than 8 feet should be avoided unless the rivers are very small. Casting becomes less accurate with a small rod which offers very little advantage except in very specialised circumstances. For most streams a rod between 8 and 9 feet in length is ideal and rated AFTMA 3-4. It is very easy for most fly fishers to cast a line a weight lower than the rod's rating and gain an advantage over the fish in terms of stealth. Other than this I will not offer any further advice on rod choice because it is largely personal.
Dry Fly Lines
Fly Lines for river fishing should be double taper and floating however these are becoming scarce. Manufacturers prefer to market ever more innovative weight forward lines with slim forward profiles. Ironically these look like double taper lines. Why double taper and not a long slim tapered weight forward? A weight forward line might be easier to cast further however on a river range is rarely a problem. Indeed most capable casters can still put a long line out wether double taper or weight forward. In other words a weight forward line offers no real advantage to the stream fly fisher.
On the other hand a double taper offers a considerable advantage in terms of presentation. The weight is distributed so that the lightest part of the fly line is closest to our dry fly. Fly lines do carry weight. If the heaviest part of the line is landing near the trout (as in a weight forward profile), we increase the risk of letting the trout know we are there. The real reason manufacturers prefer us to buy weight forward profile lines is this. After one taper on a double taper has been well used and past it's best the line can be reversed. Essentially a double taper is 2 fly lines in one.
Lines are rated based on weight using the AFTMA system. This runs from line weight zero (almost weightless) to 15 (heavy climbing rope). This is the weight of the fly line. If you have a 9 foot AFTMA 4 weight rod you can still fish a 2, 3 weight line (reffered to as underlining). A 4 weight line will load the rod optimally with 30 feet of line out. You can also over line the rod and fish a 5 or 6 weight line although this offers no real advantage in most cases. A double taper Floating 3# line would be my personal recommendation for fishing on a 8-9 feet 4 weight fly rod.