Translation:The boy comes home at four o'clock.
The form depends on the last word in the expression for number. The system works as follows when you combine a noun with a number in Nominative (or Accusative that looks like Nominative):
If it is один, you use час.
If it is два, три or четы́ре, you use часа́ (this is a counting form different from usual Genitive ча́са — a very small number of nouns have this feature)
If it is пять, шесть and so on, use часо́в
через + Accusative. Here are a few examples:
- через 4 часа = in 4 hours
- через год = in a year
- через два дня = in 2 days
- через неделю = in a week
This is covered much later in the tree (Time expressions 2, I believe). For now, simply saying time by the clock is good enough.
According to openrussian (https://en.openrussian.org/ru/%D0%BF%D1%80%D0%B8%D1%85%D0%BE%D0%B4%D0%B8%D1%82%D1%8C), приходить means "arrive". So why is wrong "the boy arrives home at four o'clock"?