Well, reading these lines from the book, I don't think Mordor's corruption was cleansed anytime in the Fourth Age.
Quote:
They had come to the desolation that lay before Mordor: the lasting
monument to the dark labour of its slaves that should endure when all their
purposes were made void; a land defiled, diseased beyond all healing --
unless the Great Sea should enter in and wash it with oblivion.
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'unless the Great Sea should enter in and wash it with oblivion'. That would mean the Bibical Flood that Tolkien predicted at the end of the Fourth Age.