1. On the ground floor of your house. On the leftmost shelf in the room.
2. In the garden behind your house, in the upper right corner, there is a pile of sacks. You will find the letter there.
3. In the garden, on Day 2, you will find Elio's third letter on the table of books.
4. On Day 3, in the garden of your house, you will find the fourth letter in the lower right corner.
5. In Elio's secret room, inside the well, on Day 4 if you have Elio's Ring and the Dice Set.