Confusion over the combination to the key

[Crowd]

Regarding the combination needed to open the box in the dormitory, the one with what I assume is the key to the archive..... I have found the the two hint-papers with numbers, but I can't seem to figure out what I'm ment to do with them... I found the place where I'm supposed to insert a code, but neither codes works and adding them together or subtracting the lesser number from the bigger one didn't work either.... I've tried all kinda of combinations I could think of, but I'm out of ideas of what I'm supposed to do.

[сенсей :3]

find a list of posters and follow them
then use the posters, look at them very carefully

[Crowd]

Oh.... is it that kind of old-school point and click logic? I didn't see anything that indicated that the poster note was related to this set of numbers.... or maybe I just overlooked it. Thanks a lot!

I managed to make sense of at least one of the codes (the 4213 one), the one that unlocked the achievement and some kind of blue star? But I can't make sense of the second code... Speaking of the blue star, when it showed up I thought it was an item, but it doesn't appear in the item-list, so is it just a "gold-star" for figuring out the 4213 code/getting the achievement?

(Spoiler for the logic of the puzzle) I think I initially missed that there was numbers on the posters because I didn't pay that much attention to the details of them + I played in windowed-mode rather than full-screen, and that kinda made a lot of the numbers a lot easier to miss. But when I started looking they where reasonable easy to find, the only one that is giving me trouble is poster "6" ("Don't be afraid of the new world, be afraid of the past"). I can't find a number in this one.... unless it's one of the cracks? One of them kinda looks like a 2, but the combination 8251 didn't give me any results when I used it.... I even tested to brute-force it by guessing the last number, but still didn't get it...