My own preference for any conditions where the size of the flies, the amount of wind, and the required length of casts permit, is a 7'6" for 3 wt. The one I am using currently is a Winston Ibis. Thinking about going with a different maker / type of rod in the near future, but in the same length and line weight.

The pros are that this length and weight rod ( your 7'6" for 4 wt ) will handle just about any fly you want to use, including some fair sized weighted nymphs, probably up to size 6 4X long, and good sized dry flies, up to size 8 2X long stoneflies and hoppers, for example, on mid size rivers, and should cast reasonably well in a breeze, if not a gale.

The 7'6" for 4 wt should easily handle any fish you catch in a mid size river. And if you can't handle the "big one" you'll still be happy that you hooked him and you'll still have a great memory from the experience. I've landed 18" plus trouts in fast water on my 3 wt without feeling that I was stressing the rod.

The shorter / lighter rods are easier to fish over the course of a long day on the water. When I step up to my 5 wt, the difference is very noticeable. Doesn't stop me from fishing it, just is a bit more tiring after hours on the water.

The shorter rods are easier to transport, and easier to handle around close quarters.

The cons are that bigger flies and tandems of weighted nymphs, stronger winds, and longer casting distances don't mix well with the shorter / lighter rods. Some of the really good casters can do with a 3 or 4 wt what a lot of us might prefer a 5 or 6 wt for, but those guys are the exceptions.

John