1. Hurk's fourth letter
Go down into the bunker shown in the screenshot, which you will find in the area marked as number one. Inside, go down the stairs. On the right side of the chamber, you will find the letter.
2. Hayato's fourth letter
Follow the path into the area marked as number two. There, you will find a vine that you need to use to climb down. On the right, at the bank, you will find the letter.
3. Masawiro's letter
In the area marked as number three, there is a corpse with the letter (the screenshot), next to the building's entrance.
4. Hayato's third letter
Reach the town marked as number four, where, inside the bunker shown in the screenshot, you will find the letter.