> 4-letter-long and above prepositions are capitalized. Can't remember where I got that rule, but they're probably using that; I use it for my music collection.

This is the guide I follow, and it would only capitalize preposition five characters or longer:

http://aitech.ac.jp/~ckelly/midi/help/caps.html

It's called title case, and it's not determined by the length of a word but by whether or not a word is minor. The rules defining minor words vary slightly between styles, but they're usually prepositions and articles.

You and APA seem to disagree on this https://apastyle.apa.org/style-grammar-guidelines/capitaliza... but it's ultimately one of semantics since if your list of minor words are all three letters or less then it's the same.

> Lowercase only minor words that are three letters or fewer in a title or heading (except the first word in a title or subtitle or the first word after a colon, em dash, or end punctuation in a heading)

I learned it as all prepositions and articles, unless they start the title.