The makers of season 4 seem to have listened and brought back what was missing from season 3: the mystery being solved layer by layer, Harry's detective skills, the beautiful northeastern settings (this time, King-esque Maine), and great leads in the center of the mystery.
This time, it's not just "why-dunnit", but "why-dunn-what". Harry, being a witness himself, is not even sure what he saw, and recovering from the events of season 3 is not helping. Already retired, he is pushed to join the investigation because the local police simply lacks the skills to deal with serious crime. Sonya who is still with him (possibly the most annoying character from season 3), thankfully is barely there. So the focus is mostly on the mystery and the usual little town secrets.
Highly recommended.