Translation:She has four older sisters.
When using the counter 人 it is read as にん (except for one and two where 一人 is read as ひとり and 二人 is read as ふたり respectively).
四人 then would be read as よにん. Here 四 is read as よ instead of よん.
So, what you're actually hearing is "Yo nin" and the last "i" belongs to "imasu". :)
I am curious about the pronunciation of 人 here. With lower numbers e.g. 二人 it's spoken futari as a special number. When we talk about a person's nationality e.g. 日本人 it's pronounced jin. But in 四人 it is nin. How do I know when it is "jin" and when it is "nin"? Is "nin" only for counting? Are 一人 and 二人 the only special cases and all higher numbers of people use "nin"?
Welcome to the fabulous world of kanji readings. ;) Well, in this case (jin versus nin), my answer would be: you have to know! And as Stephanie290249 said, you are right about the first two numerals’ being irregular; you have a chart summing this up in unit Home, along with some other counters.
Me too! I’ve started taking screenshots, it’s incredibly frustrating. For me it’s only the listening ones, I’ve noticed with those they don’t like answers with kanji except for the numbers/counters. かれ／彼 and かのじょ/彼女 however are hit or miss so I can never guess which to use because sometimes they want the kana and sometimes the kanji
I would recommend to stop thinking about subject in japanese, the grammatically subject in english and what's considered a "do-er or be-er" in japanese work differently.
Instead think about the most primitive structure of japanese [do-er/be-er+が+predicate] and the function of the topic marker. Think of what you want to say and what it means.
Let's clear the particles then, は is used to mark a topic, and a topic is something that is already known by the listener and the speaker, and it's only something you want to say something about it.. When you see は in japanese you mind should instinctively go into "what about it?", what is important is what the speaker is saying about the topic not the topic itself, は then throws the focus into the other part of the sentence.
が however is an identifier of something new or interesting, or something that the listener should pay attention to, you brain should always go into thinking that anything marked with が has an emphasis.
So now that we kinda understand both particles lets create the sentence. You want to say "She has four older sisters", what's the topic? the topic is what you want to talk about it or what you want to say something about, the topic is "she" because "she" is not important, what's important is what you wanna say about her, and what is that interesting information you want to say? that's the number of sisters she has, the "be-er" is marked with が, this is "older sisters", there are four of them.
as for her(は), she has four older sisters(が) or「彼女はお姉さんが4人います」