Ni-kai no Youkai Oujo 4

is the fourth volume of the Ni-kai no Youkai Oujo light novel series, written by Ryuji Arai and Illustrated by Kantaka. It was released under the Dengeki Bunko Imprint in May 2006.

Synopsis
The princess who bundles the demons, the superhuman Kongo demon, the thunder beast girl Rye who controls electricity, and-just a high school student Yukito. The "Mysterious Case Counseling Center" opened by the four of them is full of requests for investigation of mysterious cases today! What is the identity of the fighter plane during World War II that suddenly appeared in the mountain village? What is the mystery that attacks the Yukito who appeared? What is the secret of the dumpling shop that makes Rye who is proud of cooking? The popular youkai comedy series, this time we will send it in a luxurious three-piece stand.

(source: Amazon JP - Translated)