4 Blackbeard and his crew got close to other ships by A learning about the ship's crew. B not showing any flag at all. C stealing the other ship's flag. D showing the same flag as the other ship.

The text explains Blackbeard's tactics: "They often learnt a ship's nationality and then raised that country's flag in order to appear friendly. This way Blackbeard's ship could get close to the other ship."

Thus, the answer is:

D showing the same flag as the other ship.