Brackets indicate an editorial change that avoids changing meaning, but is necessary to obey rules of grammar. In this case, a preceding clause was left out to highlight the remainder of the sentence, so the s had to be capitalized since the remainder was being quoted as a sentence in its own right. A four-period ellipsis is usually used to indicate omission of one or more sentences, or the end of a sentence in a quote (at least according to MLA standards), while the more common three-period ellipsis indicates only the removal of words from a sentence, not of entire sentences.
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