Volume Four of Blue Exorcist begins with Rin using his powers to protect Shiemi and nearly getting discovered by Ryuji. Rin manages to evade Ryuji’s questions (barely). Fortunately, everyone is distracted by the discovery that this particular class competition will involve teamwork after all. (The initial assumption had been that the competition was an individual one. I strongly suspect that at this stage of their training, exorcists of this order get presented with a lot of activities that turn out to be team work centered even if they are framed as solo activities. This is an interesting training method.)
The kids figure out how to carry the lantern (which turns out to be a type of demon) back to the campsite. On the way back, they are attacked by a very large moth demon that is extremely unhappy with them. The monster is defeated and Ryuji scolds Rin for being bad at teamwork. (Rin has a very bad habit of trying to protect people by using himself as a meat shield. The repeating theme of this series is “don’t be a self-sacrificing hero, dumbass.”) Then Amaimon turns up again, attacking Rin. Amaimon’s attack is foiled by Shura.
We get a brief aside between Amaimon and Mephisto. Amaimon at this point is pretty annoyed about getting clobbered and wants to kill all the humans. Mephisto however states that if Amaimon kills any of them, he will kill Amaimon. (It appears that Mephisto is much more powerful than Amaimon.) Amaimon immediately backs down, stating he will “control” himself.
Back at the campsite, everyone gets ready for another possible attack from Amaimon. Shura nearly gives away Rin’s secret identity while setting up various protections. Despite the protections, Amaimon is able to take Shiemi hostage. (It turns out that during an earlier attack, Shiemi had been taken over by a parasite.) This causes Rin to charge off after Shiemi. The end result of the attack is that everyone discovers that Rin is the child of Satan.
Needless to say, this is not a good situation for poor Rin.
Rin (and Mephisto) end up on trial. Fortunately for Rin’s continued survival, the “Grigori” come to the decision that they will allow Rin to continue to train to become an exorcist. Unfortunately, he has six months to pass the certification exam. There are a lot of other conditions, but we don’t get very much detail on what they might be. Another problem for Rin is that now that everyone knows he is the child of Satan, his relationship with the other kids in his class changes drastically.
The volume ends on a somewhat sour and unhappy note as Yukio worries about his brother and Rin’s classmates try to figure out how they should react to him now that they know his secret. In the very last scene, Yukio and Shura are alerted to a dangerous infestation of demons in a housing complex in “north True Cross.” Yukio is not certain about what to do to help the situation, since he is supposed to be keeping an eye on his brother. Shura however points out that they can take Rin with them.