Halfway through season 4, Slow Horses has finally revealed more about its antagonist, Frank Harkness (Hugo Weaving), as well as his possible connection to Slough House agent River Cartwright (Jack Lowden) and former MI5 operative David Cartwright (Jonathan Pryce). In Slow Horses season 4, episode 1, two violent events unfold. First, a man using the MI5 alias Robert Winters ignites a car bomb in the WestAcres shopping center. Later that evening, an assassin breaks into David’s home and pretends to be River, prompting the elder Cartwright to shoot and kill his grandson’s look-alike.
https://screenrant.com/slow-horses-season-4-frank-harkness-plan-river-connection-explained/