The answer to this meta are the 4 letters that go in the black boxes to make valid words. Letters in the solution are ordered left to right, top to bottom.* They are all 5-letter words, and none are the letter S
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Note: you may come up with a solution using obscure words that I didn't consider, so try to find solution containing more common words ;)
* top-left, top-right, bottom-left, bottom-right.