DWM ~ you must always keep in mind these fly fishing forums are long famous for hair splitting on subjects and different weight lines for fly rods happens to fall into that category! (LOL)

4wt is as small as my fly rod arsenal goes (runs from 4 to 9wt). Since I'm far from the sharpest knife in the drawer, I try to keep things as simple as possible so at least I can understand it!

Some years back I attended a Trout Unlimited meeting that had a great speaker from a rod manufacturer (St.Croix). He talked about the manufacturing of fly rods and tossed in a few other tidbits to boot. Some of the main things I picked up from the talk was that fly rods should be able to handle (in addition to the recommended line weight) going up or down at least one weight size; and, of course, affecting the rod's performance, too. What I gleaned from it for my use is that by dropping down a size does better for a shorter leader with the line alighting on the water softer. Going up a size makes for a little better distance casting AND for the short casts will handle a longer leader.

Let me add ... although I presently have about 13 fly rods, I try not to do the multiple weight rods but I just counted three that are. For example, my favorite (like kids, your not supposed to have one!) rod is the 5wt - Do NOT want a 4/5 or a 5/6, I want a FIVE WEIGHT. Might get too complicated for me!