Question about plumber's responsibility for a leak that didn't show up right away

You are. No written contract means no warranty.
Even with a contract, would the plumber warranty their work for four years? That's an awful long time.

There is no way to PROVE that the problem stems from the original install.
You said it was an honest mistake from an otherwise good plumber. To keep the peace, and still be able to call on this plumber in the future, I would pay him his bill, pay the repair costs because of the lack of a contract and warranty, and move on with life.
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