Volume 4

Incredulity Time (まさかの時間) is the fourth volume of the Assassination Classroom manga.

Publisher's Summary
English teacher Irina's assassinationmentor makes an appearance and the two compete-- using special agent Karasuma as their target. Another transfer student/would-be assassain joins the class, accompanied by a guardian who claims to be Koro Sensei's... little brother?! A tentacle showdown ensues, sending everyone into a tentacle tizzy. Then, the 3-E students face humiliation yet again during a schoool basball exhibition match. Plus, more secrets of Koro Sensei's mysterious past revealed...!.