It is rare and difficult for a long-running series to reverse a downward slide. There are simply too many sources of gravity: writers run out of obvious plot lines and have to start re-hashing, Flanderizing, and escaping the confines of the show's existing logic; actors change, and audiences grow tired of what was once novel; it is simply hard to capture lightning and put it back into the bottle, and the commercial demands of the television industry incentivize showrunners to turn their works into shambling ghouls (another year of steady work for the crew, another year of solid residuals, another year of fun with coworkers with whom you've grown fond) rather than a surgical and cohesive narrative arc.

(All of these things are true of books series as well: take, for instance, the Berlin Game nonology which started with promise and ended with Deighton left with no other choice but to raze his characters [and goodwill] to the ground.)

Slow Horses bucks — or at least bucked — this trend. Its fourth season is a triumph: it rights every slight misstep of Slow Horses (Season 3) and Slow Horses (Season 2) (both solid seasons in their own right, but not nearly as great as the gemstone first season) and delivers the best six hours of thrilling, compelling spycraft in a long time.

Some miscellaneous notes:

• One of the keys to Slow Horses' success is the legitimacy of its stakes. There's no Game of Thrones-esque flippancy with which characters are killed off less out of narrative cohesion and more out of a desire to shock the user, but nobody (besides, presumably, Gary Oldman's Jackson Lamb) is safe from death: this is something we're reminded of every season, and it gives a sense of actual danger and urgency to every shootout or chase.
• Speaking of Jackson Lamb: this season probably had the least screentime from Oldman. This can be a dangerous gambit — it paid off here, and his absence (modulo the handful of convenient deus ex machina opportunities he's afforded) is felt but not painful.
• Spy stories must thread a very tricky needle when it comes to realism. It's really hard to be entirely grounded in craftwork and realpolitik unless you're le Carré or The Americans, and it's also hard to be bombastic and exciting in the traditional language of television action sequences without feeling like you've suddenly entered a different, sillier show. (This was, I'd argue, the main flaw in Season 3: we suddenly entered a Call of Duty commercial in the back third of the season.)
• Season-long arcs dedicated to flushing out a character's backstory can, similarly, be hit or miss: shows need to earn an audience's trust that we actually care and empathize with them enough to follow them down unexpected corridors, and actors need to be able to shift from being relatively one-note to full-fledged protagonism. Jack Lowden passed with flying colors.

★★★★