Black Francis gives great advice.
For small rivers, dries and small wets, trout of reasonable size with 40-60ft casts in mild wind the 4wt would be a pleasure. However if the 6wt angling friend fishes the same waters, one would have to assume there is a calling for the bigger stick (?)
It's hard to guess without knowing your waters but it's readily apparent why the 5wt fits so well into the trout fishing niche.
Maybe since the seller has a 4wt of his own he might lend it to you for the day you could take it for a road test and make a judgement on your own.
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