Fantastic Duo 4/4- After getting the first 2 on Tideshelf Path and the 3rd at Nia Village, Teleport to the triport at the Starlight Shelter and head East. If you look on the map, theres like a "S" looking shape near Inath Beach. Head to that "S" shape and you will find a hammock tied to 2 trees. The 4th and last painting is located near the bottom tree.