What Should I Do Now?

 is the fifth and last chapter in the fourth volume and the eighteenth chapter overall.

Summary
The chapter opens with the 104th Trainees Squad still in basic training. During a march, Armin is having a hard time keeping up with everyone due to the weight of the pack. Reiner comes up behind him and takes his pack for him, without the instructor noticing.

In the woods, the trainees are being tested on their ability to use the 3D Maneuver Gear by attacking wooden cutouts of Titans. The competition gets pretty fierce, with students snagging the killing blow from under others. The instructor takes notes of the students individual strengths and weaknesses. Now back at camp, Jean complains about stolen targets. Eren notices that Marco barely attacked the targets. He explains that he thinks of a real life situation, using himself as a decoy while others make the killing blow. Eren says that he is a good leader, with Sasha and others agreeing. A comment from Jean nearly causes a fight between him and Eren. Fortunately, the others are able to diffuse the situation. When no one else is around, Marco tells Jean that he thinks he would make a good leader. Jean disagrees, but asks why he thinks so.

Now back in the present day, Jean is assigned to clean up dead bodies in town along with others. While lifting one body, he looks over and sees the body of Marco. He's been half-eaten by a Titan. Shocked, Jean tries to find anyone who knows about Marco's last moments. A nearby soldier is taking the names of the dead, and asks Jean if he knows this person (referring to Marco). He pays little attention, which causes the soldier to say that right now their duty is to clean up the dead bodies before they contaminate the city. Jean, still at a loss, gives the soldier Marco's name.

In another part of the city, Sasha comes across a pile of partially eaten dead bodies. Not knowing what to make of it, another soldier tells her that Titans don't digest their food, when they get full, they regurgitate. Annie sees a dead body and asks for forgiveness. Reiner, tells her that apologizing won't make any difference. In a hurry to get the memorial service over with, he thinks back to all the training they've gone through only to have this happen. At the memorial service, most of the remainder of the 104th, stand in front of the funeral pyre. As Jean crouches, memories of Marco, Eren and others come to mind. He regrets joining the military only because he worries about which of his friends will be next. Announcing it to his friends nearby, he decides to enlist in the Survey Corps.

They spend an entire day cleaning up the Trost District. During that time, the cannons never stopped firing on the Titans that had gathered at the wall, even managing to capture a couple alive. Eren wakes up three days later, after having been in a coma from his last Titan transformation. As he sees Commander Erwin and Levi, he realizes that he's locked in a cell underground. Commander Erwin begins to question him and shows him the key they found on his necklace. At the sight of the key, he's asked if it can give them the answers. Eren explains that if what his father said is true, then it will. Levi comments on the convenience of losing his memory at around the same time that his father goes missing, causing Erwin to correct him. Erwin then asks how he wants to go about getting to the basement. To do that they would have to recover the Shiganshina District and plug the hole in Wall Maria, which unfortunately would prove to be a difficult task. When Levi impatiently asks the same question, Eren, remembering his family and friends that have been killed, replies that he wants to kill them all. Amused, Levi says that he will vouch for him to join the Survey Corps adding that if he goes on a rampage that he will be able to take him down.