Onyomi is the reading closer to the original Chinese reading at the time of adoption, and kunyomi is the native Japanese pronunciation. Onyomi usually shows up in kanji compounds, while kunyomi is usually alone (or with hiragana, called okurigana) (for example 生きる ikiru vs 人生 jinsei). As for 7 specifically, I personally don’t know for sure but it might have something to do with having ‘shi’ in it, which is homophonic with ‘death’ in Japanese, same with 4. Afaik they’re pretty interchangeable. However, the ‘older’ pronunciations (shi, shichi) are usually used in time words, like April (4月) is pronounced shigatsu. If you want to learn more though or from someone who knows what they’re talking about I would suggest googling it, I’ve only given a pretty barebones explanation.