Learn Russian: Я читаю этот текст четыре часа. - I have been reading this text for four hours.

This sentence uses the rule you are learning right now! After the numbers 2, 3, and 4 (like четыре), Russian nouns take the Genitive singular form. The dictionary form is час, and its Genitive singular is часа.

Why does the Russian sentence use the present tense читаю when the English translation says "I have been reading"?

In Russian, if an action started in the past and is still happening right now, you use the present tense. Because you are still reading the text, you simply use Я читаю (I am reading), even though English requires "I have been reading".

The English translation says "for four hours", but I don't see a word for "for" in Russian. Why?

When stating how long an action takes, Russian usually does not use a preposition like "for". You just say the time phrase directly. So, четыре часа literally means "four hours", but it functions exactly like "for four hours" in this context.

Since "this text" is the direct object being read, shouldn't этот текст change its ending?

It actually is in the Accusative case, which is used for direct objects! However, for masculine inanimate nouns (objects that aren't alive, like текст), the Accusative form looks exactly the same as the basic dictionary form.