The maximum, as provided by general consensus are 4 letters. The fifth letter, when applicable, signifies company status. "Q" for bankruptcy, "A" or "B" for class names, "E" for delinquent, "J" and "K" for voting and non-voting respectively, and so on. See https://www.investopedia.com/ask/answers/06/nasdaqfifthlette... (doesn't just apply to NASDAQ)