Translation:The boy comes home at four o'clock.
How to say "in four hours"? Like "it's 1PM, he will be here at 5PM, so he'll come here in four hours"?
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.
Probably the most economical way is to just switch to the perfective version of the verb: Мальчик придёт домой в четыре часа.
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
And a little exception for 11, 12, 13 and 14 - we also use часо́в here.
- 101 - час
- 102 - часа
- 111 - часов
- 112 - часов
Igor was talking about the words in the name of the number, not the numerals, the teens don't count for this reason.