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.

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 )

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

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.