From a creative point of view, the answer is simple:
There are no fundamentals to learn. None.
You can start literally anywhere - with the sound of a particular instrument in your head, with a mathematical formula, with a picture of a cactus... and go in any direction that ends up creating sound (or not!).
By all means, you could go down the direction of learning an instrument, learning theory, and so on. You'll have to work very hard to get far down that road in four months... but you may get far enough, and having a "beginners' mind" is often seen as a very good thing from a creative point of view.
Or you could come up with a soundtrack concept based on getting people together and ad-libbing whistled themes. Or you could make a soundtrack based on sound effects, or made from cut-up samples of pieces of audio already in the (legal) public domain. Or based on having people you know phone you and leave bits of sound as messages, which you work into the film...
...or a combination of all of those things.
Don't neglect all the stuff that may seem 'secondary' but will be an important part of the project, like the recording technology you'll use, mixing the tracks, syncing them to the picture, matching the mood of the music to the story, and so on. You could learn a lot from just taking pre-existing pieces of music and effects and placing them against an existing video. Learning all that in 4 months could be a tall order, so don't focus too much on the 'pure' musical side.
Sounds fun. Good luck!