At the end of Season 3, Barry sacrificed himself and went into the speed-force. He fought with himself in the form of Savitar, and it was the challenge of speed for him.
Obviously he has to come back, and he will. This time, having stayed in the speed force, he will have some new powers, and hopefully the emotionally strong barry allen will emerge.
The most interesting thing about this new barry is his Suit. I personally like this new suit more. This is the suit Future barry had adopted.
In the season 4, there will be a whole new kind of villain. Known as “The Thinker”. This will make the whole Flash season somewhat different. In the past Barry had to have more speed. His sole focus was on gaining more speed. In this season 4, the rules will be different. Barry being the fastest man in the world wont be able to defeat The Thinker.
The Thinker is the man with fastest mind. Barry will have to outthink him. This new challenge will be quite stressful for Barry. And he will have to face really tough times and lots of failures. He will be heartbroken and often feel helpless. Eventually he will realize that speed is not the only thing in this universe, and will correct himself to focus on what matter. The Mind Game!
With these new challenges, I see a strong character of Iris West. Her role will be important now. She will have huge influence on Barry in coping with the tough situations. She will help him focus on right things and motivate him to take on challenges coming. I also see them in an improved relationship, with more trust and dependency.
Wally West will also be a grown version of a speedster. He had been happily responding to the Kid Flash image. He has got enough experience now, and has seen situations he couldn’t imagine at first. He was stuck in Speed force, and sure it had an impact on him. Looks like Wally will be a serious kind of speedster now, with some new name. There are theories by fans and many bloggers suggesting that Wally will have a lead role now, and will replace Barry as the main speedster saving the city. — I doubt it, but still believe that his role would be of some serious kind now.
Another important character that we all love is Caitlin snow, a.k.a Killer Frost. Many fans only want to see her as Killer Frost. The villain role on her is really exciting and is more energetic than a lab scientist Caitlin Snow. In season 4, both kinds of fans will not be disappointed. Caitlin will have a balance of two. She will have dual personality in this season. This will be doing great positive things and will also be equally evil at times. In the coming up suicide squad 2 movie by DC, Killer Frost will be the part of suicide squad. The TV show might have more evil role now than the good Caitlin Snow. In any case, she will be fun to watch.
Since we are having Supergirl, Arrow, and Legends of Tomorrow crossovers, There will be even more crossovers this season. Its been discussed that there is an underground fight club in Flash, just like they once had in Supergirl. Barry The Flash and Oliver Queen The Green Arrow would be fighting in that. There may be more characters involved. To what extent the club would operate and what impact it could have on our lead players, we will witness it soon.
Another thing circulating is that Barry will become Black Flash, a kind of Villain. Probably it will be the Nazi Flash, just like they did it in Supergirl and made it Soviet, now they will choose Nazi instead of Soviet. How will Barry become one and what would he do? Could it be Barry from different timeline or something like Savitar? The Flash team is known to work 12 hours a day during shoot. They are trying their best to make it interesting, and im sure they will do it!
In the Promo we also see Cisco using a new kind of speed gun that will bring Barry back from speed force. Cisco has been thinking more about himself and the people around him. Hes definitely not the same fun-loving Cisco we used to have before the flash point. He will have more power now, and he would be knowing even more about using his powers the best way he could. Although, I personally loved the old Cisco. The new always-in-serious-mood Cisco looks a bit boring. But in the end of season 3 he was improved. I hope he will be similarly joyful now.
With Harrison Wells, the HR, sacrificing himself, who will replace him? Well, they still have another Wells alive and well. He may move to this earth now. Or, they might invent something else. But HR had been a key player in all seasons of The Flash, they can’t simple just let him dead.