Season 4 of “Slow Horses,” which aired in the fall of 2024, was a showcase for former MI5 officer David Cartwright, the grandfather of agent River Cartwright and a man whose murky past in the espionage service included enough underhanded dealings to have earned him the nickname “the Old Bastard.” Played by the formidable Jonathan Pryce, David appeared in most of the season’s episodes and played a crucial role in River’s discovery that his father was a brilliant and deadly mercenary, all while