"Βασιλεύς Βασιλέων, Βασιλεύων Βασιλευόντων" translates to "King of King, Ruler of Rulers". I know it was a "motto" of sorts in the empire (during the Paleologos dynasty) but I couldn't find a source if the 4 B's actually stood for that.

(Also, for the longest time, I thought they were crowns and not B's)