There is a unique, well-defined word that completes the following quadruple:
Ewan, Ries, Avut, ????.
What's this word?
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"Toba", the last four letters of the other province/territory that meets at the four corners (60°N, 102°W).
That said, the question says the answer is unique, but there may well be other quadruples of Canadian provinces/territories that make sense and include the three in the question. Perhaps a foursome that produce some natural resource, or a foursome that have population within a certain range, or a foursome whose largest cities have some letter, or…. I haven't checked, but I suspect there's something.