I am going through the difficult task of buying my first single handed trout fly rod. I have some experience with the double handed salmon rods. Some of the trout rods appear to have a "Fighting Butt". For what size fish does this feature come into play? I will be fishing for trout from between 300grams to 4lbs.
A fighting butt is not necessary in a fly rod when fishing for smaller fish that you mentioned. The fighting butt comes into play and will be more useful with the larger fish (i.e. steelhead, salmon, saltwater fish) Most fly rods that are sold in the 3-5 weight class don't have a fighting butt anyway.