Reiner has always been a particular character in The Attack of the Giants. Both in the manga and in the anime, his presence was fundamental for Eren Jaeger and the other companions of the legion, both before and after the betrayal. With the ending of The Attack of the Giants 3 part 2, he was forced to return to the motherland, fleeing.
So we see him still alive in the trailer of The Attack of the Giants 4, where he is on an airship with Zeke Jaeger and ready to launch an attack against a foreign power. But Reiner will also be in the center of the rest of the trailer with several dedicated scenes, one of which is particularly dark. Although less clearly than the manga that makes the weight of the scene even better, there is a short piece of the video where Reiner is in a cellar.
He is seen seated, with the light coming from behind, while he holds with his two hands a rifle pointed directly inside the mouth. Reiner is therefore trying to commit suicide, although without success, for the weight of what he has done in recent years. This shows how completely the warrior's psyche is completely shattered and his contribution in The Attack of the Giants 4 is still under discussion.
Despite the choices made, Reiner remains a character loved by fans, who are wondering what fate will be for him in Attack on Titan 4.