Here is a guide on how to find all hidden statues. SPOILERS!
NEPTUNUS
On the left side between first and second aqueduct:
At the end of main road, to the left of third aqueduct:
On the right side of entrance to the third aqueduct:



TELLUS
While solving bridge puzzle at the farthest end of this map:


SOMNUS
On the left side when you arrive to the big lake:



MINERVA
Far on the left side when you start this level:



Thanks to DDtective for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.