Back to the Joe Humphreys reference, the hot "light" rod in the 70's was the Fenwick 756 - 7 1/2' 6wt. Broke out my 756 a couple of years ago at the Little Lehigh during trico season and the soft fiberglass rod really delivered those tiny flies well.
Like most said, I usually fish dries with my 4 wts. However, add a little wind or a little distance (i.e. Delaware system) and I'm sure glad I have my 5 wt or even a 6 wt. Also, larger bugs like drakes or isos may be better presented with a 5 wt. Both heavier rods do just fine at shorter distances. Modern rod technology is such that 5 and 6 wt rods are lighter than 4 wts were 25 years ago so you shouldn't get tired waving the bigger stick.
But my 4 wts are sure sweet tossing size 16 sulphurs on the mid-sized streams I normally fish so that's why it is my usual weapon.