In the original English series, this fourth installment is titled " Missing Linc " . It follows the story of Linc, a teacher, and a student in a "bi-awakening" MM (male/male) romance.

You can find more details or purchase the series on Amazon or the author's official German page . Rhys Everly (Author of Insatiable) - Goodreads

Series Overview: "Für das Leben lernen" (A Proper Education) Beau Pair Book 2: Drei sind einer zu viel (Original: Me Three ) Book 3: Dein einziger Fan (Original: Your Only Fan ) Book 4: Eine Lektion fürs Herz (Original: Missing Linc )

The book you are looking for is titled (German for "A Lesson for the Heart"), which is the fourth book in the "Für das Leben lernen" series by Rhys Everly.