Saw Gerrera is one of very few characters in the Star Wars saga to have a role in the Clone Wars TV show, Star Wars Rebels, and a movie. His appearance as a grizzled extremist mostly made of machine in Rogue One was a far cry from the bold young man of The Clone Wars, and it was shocking to see how far he had fallen. Saw is coming back to the action in Star Wars Rebels Season 4, and showrunner Dave Filoni's description of his part to play indicates that we may have to change our minds about Saw the extremist. Filoni had this to say:
Saw's always seeking his own path, and he's certain of the evil the Empire's up to and there can be no peace. He's been pushed even further down that path of belief. [Alliance leaders Mon Mothma and Bail Organa] still hold out hope the Senate can find a peaceful way to resolve everything, and it won't come to all-out war. Saw is trying to get the evidence and prove out that he's right. We have some fun scenes when he interacts with Mon Mothma and it shows the fractures between the two.
We already knew that Rebels Season 4 would delve into the conflict between Saw Gerrera and Mon Mothma that was seen in Rogue One, and Dave Filoni's comments indicate that we'll get to see Saw's side of the story more than we did in his prior appearances in Star Wars stories. When he turned up in Rebels Season 3, he was already far more extreme than the Ghost crew, and it was somewhat difficult to sympathize with him. It sounds like we'll get a different perspective on how Saw became what he was by the time Rogue One came about.
Fans of the Star Wars universe know that Saw is actually right about the Empire working on an evil plan that will make war inevitable. Now that the Rebels timeline is fast-approaching the events of Rogue One and A New Hope, the Death Star is in its final stages of completion. Saw's upcoming attempts to find evidence about the Empire's activities are doomed, given the events of the films, but we'll be able to sympathize with him as he tries to warn the Rebellion that war is coming. The extremism that backfired in Rogue One was evidently brought on by the Alliance's refusal to believe Saw's claims about what the Empire has brewing. While his actions in Rogue One--especially with regard to poor Bodhi--won't necessarily be justified, they'll be more understandable.
There's also the point that Saw's rapidly deteriorating health might contribute to his extremism that was on display in Rogue One. He was a fit and able man when he appeared in Rebels Season 3, but glimpses of Saw in the trailers for Season 4 indicate that something has happened that his health has taken a major turn for the worse. His hair seems to have gotten its white streaks in the short span of time since he last interacted with the Ghost crew. The speed of his decline could contribute to his increasingly extreme tendencies.
Although we don't get full body shots to show if he's lost limbs just yet, we can bet that he won't be in great shape when he turns up. If we discover that he was somehow infected by something during his mission on Geonosis, that would certainly explain his fixation on discovering what the Empire is planning. Dave Filoni went on in his chat with EW to explain that Saw will be looking for some specific allies in Season 4:
He looks at someone like Ezra and Kanan and thinks, 'What a wasted resource.' He fought with the Jedi in the Clone Wars, so he knows what they're capable of.
Most of the rebels fighting for the Alliance on Star Wars Rebels have been newcomers without ties to The Clone Wars, but Saw was in the thick of the action in the final years of the Republic, and even trained under the guidance of Ahsoka Tano (who is also returning in some form or other in Season 4). He has seen what unleashed Jedi are capable of, and he may go to extreme lengths to try and get Kanan and Ezra on his side. The new trailer indicates that he might be successful with at least Ezra.