In the Netflix reboot titled “Cobra Kai,” now in its fourth season, the original actors have returned as adults who run their own dojos, but are still clinging to their old rivalry, which infects their own karate-obsessed children. Cobra Kai’s ruthless sensei John Kreese (Martin Kove) also returns, as does his gray-haired henchman Terry Silver (Thomas Ian Griffith). Martial arts fervor sweeps over the suburbs of Los Angeles, and kids join Miyagi-Do, Cobra Kai and Lawrence’s upstart Eagle Fang as if they were street gangs. The animosity becomes so vicious that one student is literally paralyzed during a fight at...