Attack on Titan: Junior High Vol 4

From the Kodansha edition

SNOWBALL FIGHT ON TITAN As winter nears, a legend spreads through the student body about a mysterious "Snow Titan." When a sudden cold snap hits Attack Junior High, the story turns out to be more than legend! Later, what do you get humanity's strongest soldier…on his birthday? And the peace of the campus comes to depend on the wiles of the one least in touch with reality…Armin!

Source: Penguin Random House API