If Cobra Kai season 4 incorporates the primary villain of The Karate Kid Part III, Terry Silver, then it makes sense to bring back the third movie's secondary baddie, Mike Barnes. Portrayed by Thomas Ian Griffith and Sean Kanan, respectively, the characters torment Daniel LaRusso (Ralph Macchio) in the 1989 film, and also threaten Mr. Miyagi (Pat Morita) while John Kreese (Martin Kove) plots his revenge. Cobra Kai season 3 heavily implies that Silver will be part of future episodes, and so the return of Barnes would strengthen the on-going crossover with the movie franchise.
Because Cobra Kai has such a large global following, the showrunners often use narrative misdirection to tease the audience. After Tory Nichols (Peyton List) showed up in season 2 and started a season-ending school brawl, fans theorized that she could actually be the daughter of Silver; Kreese's comrade from the Vietnam War and a wealthy businessman who funded the Cobra Kai dojo. In season 3, numerous flashback sequences actually show Kreese speaking to a fellow soldier named Silver, and the season finale implies that Silver could indeed be the mystery person that Kreese calls in a climactic moment. If Cobra Kai continues to reference the movie franchise, then Silver and Barnes should be part of season 4.
Silver's return in Cobra Kai season 4 needs to happen. Now that Johnny and Daniel have teamed up and will presumably square off against Cobra Kai in new episodes, Kreese will need funding and strategic advice from his long-time pal, the man who saved Cobra Kai in The Karate Kid Part III. After developing Silver's arc through flashback sequences in Cobra Kai season 3, Griffith's character doesn't necessarily have to be a prominent figure in season 4, but a cameo appearance could help explain how and why Kreese's dojo becomes more influential in the San Fernando Valley. If Silver does indeed return, then his top recruit from The Karate Kid Part III, Barnes, should also make a cameo, if only for the sake of referential completism, and also to further reinforce Kreese as a vindictive and scheming baddie - someone who knows that Barnes once terrified a young Daniel LaRusso.
Since List has been promoted to a Cobra Kai series regular, her character Tory will most likely evolve into a proper baddie during season 4. In that case, Silver and Barnes would theoretically have smaller roles. As of now, fans know what to expect with Griffith's character - he's probably still incredibly wealthy, and also understands every aspect of Kreese's "No Mercy" origin story, evidenced by what they experienced together during flashback sequences in Cobra Kai season 3. As for Barnes, his character hasn't been properly developed in the Netflix series, aside from brief mentions via character dialogue. However, Cobra Kai season 3 does technically show him in a flashback sequence when Daniel informs his daughter, Sam (Mary Mouser), about the fear he experienced while defending his title against Barnes in the All Valley Karate Tournament. In a way, this small moment sets up Barnes' return.
In Cobra Kai season 4, it may not be entirely surprising if Tory turns on Kreese because of her humble background and Silver's elitist arrogance. The series is fundamentally about redemption, which means that List's character will need to take accountability for her violent actions at some point. If Tory betrays her dojo, then either Silver or Barnes could emerge as Kreese's top henchman in Cobra Kai season 4. Griffith hasn't actually appeared in a movie or TV show since 2007, however Kanan remains a working industry actor and currently stars as Dr. Pierce Hartley in Studio City, which suggests that he's ready for a recurring role in Cobra Kai. Moving forward, the Netflix series could further develop Tory in season 4, incorporate Silver through a cameo role, and then position Barnes as the secondary villain for Cobra Kai season 5.