I agree a 7 foot full flex 4 weight is a rush, but still a backbone. I have more fun with the 4's than I did with my Scott Eclipse 3wt.
The difference in rod material/ action in sizing your tackle for smaller fish is far more important than line wt. I once had a Scottt glass 3wt, medium fast, and probably equilivant in power to the average grafhite 2 wt. I have a glass 4/5 wt, very full flex, that is more fun on smaller fish than the Scott. Maybe it is something you have to experience for yourself.