4wt vs 5 wt rod

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There is also the issue of flex that comes into play when selecting a rod. I suspect that your ultralight experience had more to do with the flex of that particular ultralight rod. You get a good UL and get good with it, you can fish any small stream and get fish. Takes some practice, but they can be close range killers.

The same comes into play with fly rods. Slower rods flex more, they have flex through a longer portion of the rod, they lay small dries onto the water ever so gently...
Fast flex rods have the majority of their flexin the upper part of the rod, they punch flies through the wind and fire them to the edges of cover but can snatch the fly away on hook set if not careful and may break a light leader on the set of your biggest fish...
Medium flex rods are just that, they are somewhere in the middle of the extremes with each manufacturer having their own definition of medium.

Not to break with Riverdogs advice, but I would be very leery of buying a used rod off E-bay, that's just me. I like to cast them if I can.
If I was going to buy used I would check with all the local shops. Those guys are always buying new and selling their used rods.
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