Here is the full list of all 39 Trine 4: The Nightmare Prince achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Collect all Knicknacks in the game
Collect all Treasures in the game
Collect all Letters in the game
Complete A Wintery Morning
Complete The Cursed Manor
Complete A Masquerade Night
Complete Craghill Moors
Complete The Thorny Hedge Maze
Complete Heatherwood Hall
Complete The Prince's Dream
Complete Moonlit Forests
Complete The Badgerborough
Complete Goldleaf Garden
Complete Firwood Water
Complete The Crackling Mire
Complete the Blueberry Dale
Complete The Gossamer Grove
Complete Snow-Topped Heights
Complete The Nightmare Academy
Complete Haunted Tombs
Complete the game
Collect all experience in A Wintery Morning
Collect all experience in The Cursed Manor
Collect all experience in A Masquerade Night
Collect all experience in Craghill Moors
Collect all experience in The Thorny Hedge Maze
Collect all experience in Heatherwood Hall
Collect all experience in The Prince's Dream
Collect all experience in Moonlit Forests
Collect all experience in The Badgerborough
Collect all experience in Goldleaf Grove
Collect all experience in Firwood Water
Collect all experience in The Crackling Mire
Collect all experience in The Blueberry Dale
Collect all experience in The Gossamer Grove
Collect all experience in Snow-Topped Heights
Collect all experience in The Nightmare Academy
Collect all experience in Haunted Tombs
Collect all experience in the game