In addition to the X年生 series that are included on the 年別 page, there are a number of series that are aimed at 中学生. This level should be appropriate for our 4th year students, and these series usually include a good mix of literature and fiction. In many cases they are stories that would be published in standard 全集.
はじめての文学 （文芸春秋）should be in most library collections. The series includes 村上春樹、村上龍、よしもとばなな and others.
中学生までに読んでおきたい日本文学 （あすなろう書房） is a 10 volume set that is arranged by theme. This can be very helpful when students need to choose a story to read/translate and either don't know enough about Japanese literature to choose a particular author or just don't care. Volumes include villains, family, love, life, money, horror, food...
文芸マンガシリーズ is also listed on the manga/lit/anime page. It is probably the most common way for students to read Soseki or Akutagawa in Japanese these days. If you don't have it in your collection, you should probably get it and see if you can't introduce students to the originals later.
ヤング・アダルト is a fairly recent category in Japanese publishing, although it has a much longer tradition in English. It is a category used for literature written specifically for teens (13-19 years old). Instead, if you were doing research or wanting to read YA appropriate novels from an earlier period you might find them categorized as ジュベナイル (juvenile) to contrast them with 児童文学 (children's literature).