What is "trout" about a 12' 5wt? I fish a 12'6" 4/5 for fall and spring steelhead. Can't imagine a rod that big is much fun on "trout". Unless of course you're consistently catching 24+" inch fish on the swing???
It is not just the fish, but the size and weight of the flies. I have a tendency to go big
on my second run through a pool and run if there are some good pockets, holes or banks, and will catch some nice fish.
So yes, a 4wt is over gunned for most trout and small flies - which I enjoy fishing - but it under gunned for some of my heavy rabbit tube flies on that second pass.
My default rod is a Meiser 12'6" 4wt S
My travel rod for trout is a Meiser 5wt S. I forget the length, but it is over 12'. If, I am going fine when travel, then I back down to a SH rod.
I prefer a rod that is on the longer side for trout (unless, I am dry fly fishing). If I am casting under the trees (and streamers), then I move to a Scott ARC 10'0" 7Wt SH, but you will see Circle C, Double Speys coming out of my rod.
I do not use a Spey Rod for a dry fly hatch.
In short, the rod is a function of the fly and style I want to fish. Process is something like ...
* Scout and observe
* Fishing a hatch?
.... If Yes, get dry flies
.... Is river small or large?
........ If Small, probably SH Kabuto or Green 3wt - both are 7' fibreglass
.........If Large, probably Scott G 905/4 or Bamboo 7'9" 5wt (Don Anderson)
.... If No, then Meiser 1264S and get my streamer boxes (muddler, tube flies and Sculpzillas)