Do much small stream fishing for little fish? I might go towards a 3wt personally. That or might do something different like a fiberglass or bamboo rod for trout. Not sure a 6wt would open many doors if you have a good 5 and 8wt. The 4wt is similar unless it is distinct from the 5wt, like longer for nymphing or something with different action. That said, I get the 'buy more gear' instinct
I have to resist sometimes.