In astrology, the celestial bodies seen moving across the night sky (including the Sun and Moon) are thought to have a direct influence on human lives. In classical astrology there were seven planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn. Each had its own symbol and characteristics. The symbol for Jupiter looks a little like a curly number 4.
This symbol is Bruno’s personal code, used to sign letters of intelligence to Elizabeth’s spymaster, Francis Walsingham.