The four-digit number is:
This answer is terribly out of sync with the puzzle's intended solution. It turned out that many of the steps can be skipped.
Only part of this solution is my own. I rely heavily on partial solutions given in other answers and comments.
First, LeppyR64's has found out ...
... that there is hidden information in some of the letters.
Working from that:
Instruction 1 says "Towards info", which apparerntly should hint at the two 2's in the picture that guard/ward information. (The OP has explained that in a comment below.)
Instruction 2 says "Tenor" key. Decrypting the line below with the key TENOR doesn't yield anything useful, though. It turns out that (a) the key isn't really a key to the cipher below and (b) is a bit cryptic: Ten in Roman numerals is X, so the instruction says XOR key.
XOR or exclusive or is a binary operation that yields 1 when both inputs match and 0 otherwise. The two 2's can be overlayed and xored, so that pixels that are different in both 2's are white; pixels that are the same in both 2's are black. This yields:
It says "Holistically 4".
Using the key HOLISTICALLY as a Vigenère key for instruction 3 yields:
RR FROM ODD GAP
The instruction "Invert key" has the word "Lichtenstein" hidden in the first E.
Unfortunately, my solving stalled here.
Skipping several steps, Deep Purple found that ...
... the long text message can be be decodes with the Vigenère key LICHTENSTEIN. It reads:
DIGIT A WHOLE NUMBER DIGIT FORTY FIVE ROMAN RIGHT SUBTRACT THE EXTRAS IN UNDEAD CREATURE XX LOWER G LOWER S LOWER K DOT PNG
That, as the keen PSE denizen will immediately spot, is an URL on imgur. The last three characters and the file extension are given clearly:
..gsk.png. Apparently, the first two characters are two digits.
I'm not so sure about the roman numbers and the undead, but a bit of brute-forcing leads to the following image:
(Yes, cheating isn't fair, but once you've skipped some steps, you're already in the low-moral zone. Sorry for that.) The upper blank is a nonogram, as Mike Limburg has found out. Thankfully, he has already solved it:
Now the last step:
The black pattern in the nonogram is a detail from the original instruction image: It is part of the N in "Instrucions" which is underlined.
We can now overlay the pink mask so that the nonogram matches this N:
The four windows in the mask now show the combination for the safe, this time white on black. Voilà!