One of the females is unmarried. "Dear Sirs" must be out of the question. "Dear Madam / Miss / Sirs" also doesn't sound right; "dear all", "hello" or "hi" all sound to informal. As there are four of them I thought about "Dear Group", but I'm afraid some may take this the wrong way. Using their names as in "Dear Ms Black, Ms Brown, Mr Green and ... More @Wikipedia
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