enigmatic puzzle - What number is hidden in this map?

This question is part of a puzzle crime story I have started recently (you can see previous puzzles from the series HERE, HERE and HERE). I would like to test the difficulty (not sure if too easy or too hard), and possibly get some feedback/ideas for improvement. The text looks a bit out of context, but the backstory is not necessary to solve the puzzle. The goal is to find a 4-digit passcode encoded in it.

---

Sanders arrived home and looked sick. He closed the door behind him, took off his coat, and slowly went to the living room. He opened a cupboard, took out a pack of cigarettes and lit one. He quit smoking two years ago, however today he couldn’t stop himself. Sanders turned around and went to his “memory” wall – the place where he used to hang all his favorite pictures. He stopped in front of one of them and looked at it…

“Oh Teddy…” Sanders muttered and shook his head. He took the picture in his hands, sat in an armchair, and stared at it. Suddenly, he felt something attached to the back of the photo. He turned it around and saw a map.

Initially Sanders was confused – he couldn’t see written coordinates on the map or a target place on it. After few minutes of contemplating however, he stood up, put his coat back on and left. He knew exactly where to go.

---

P.S. I know, probably I won't become a world-renowned writer.

Answer

Not sure if this is right, but here's my guess.

There are some hidden

Letters in the map that seem to spell "trees". This could hint that the trees are important.

Notice that there are

10 different tree symbols on the bottom and left side of the map. This might mean they should be translated into digits.

Since the thing in the

lower left corner seems like an "origin", I assumed that we're dealing with sort of an x-/y-axis situation, and the numbers increase to the right and up. Based on where the first digit changes, we can fill in some of the numbers (again, assumption: step size of 1).

Then we have only

three remaining unused digits (4, 5, and 6) and so can fill in the rest of the map.

Then if we

overlay the photograph, we notice that the cross in Teddy's hands marks a particular box.

And taking the

x/y coordinates of that square, our 4-digit code is 5259.

cipher - What Does Igor's Letter Say?

Igor arrived at the national convention of scientists with a bit of a conundrum. "My boss gave me this message," he tells his associates, "but says he can't for the life of him figure out what it says. He won't say where he got it or anything, but he does say he knows it's a fairly well known type of code, at least in the secret code community (igor is behind the times and doesn't actually know the word cipher or cryptography, he was always more of a biology person).

I looked some of them up but couldn't really figure anything out. Unfortunately, he says it's really important for our next project so I really need to get this figured out soon."

He pulls out the letter and hands it to the other scientists. It says:

WLG CMUVGCR EVCIL LW DUPLIF BCQICTF WLG TFLVF GPDZG BYFO HOSUXJ FPRQ NEDW MP PJRX VWCQXA

"Any thoughts?" Igor asks. As they pass it around, though, he finally sees the back of the paper, on which is scrawled in a shaky, ugly hand:

ROT-5

"Oh!" says Igor, "Never mind, I got this."

What does this important message say?

Answer

The message reads

The correct brain is buried beneath the third grave back fourth from left in plot twenty.

Encoding:

This is a Vigenère cipher with keyword DECAY.

ROT-5 is a crossword-style clue to the keyword.