Translation:The boy comes home at four o'clock.
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.
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