أنتظر أربع ساعات.

Because the verb itself already tells you the subject.

In أنتظر, the prefix أ- marks first person singular, so the verb means I wait / I am waiting. Arabic often leaves out the separate pronoun أنا because it is already understood.

You could say أنا أنتظر أربع ساعات if you want extra emphasis, but it is not necessary.

أنتظر is the present/imperfect form of the verb انتظر, which means to wait.

So:

• انتظر = he waited / to wait
• أنتظر = I wait / I am waiting

This verb comes from the root ن ظ ر, which is connected with looking, observing, or expecting. In this derived form, the meaning becomes wait or await.

In Arabic, expressions of duration of time are often said without a preposition.

So English says:

• I wait for four hours

but Arabic can simply say:

• أنتظر أربع ساعات

Here أربع ساعات functions as a time-duration expression, basically four hours as the amount of time.

This is very normal in Arabic.

Because Arabic numbers from 3 to 10 usually show gender polarity: the number takes the opposite gender of the noun it counts.

The singular noun is:

• ساعة = hour

This noun is feminine, so the number must use the form used with feminine nouns:

• أربع ساعات = four hours

If the counted noun were masculine, you would use أربعة instead.

Because with numbers 3 through 10, the counted noun is normally plural in Arabic.

So:

• أربع ساعات = four hours

This is different from numbers like one and two, where Arabic often uses the singular or dual more directly.

So the pattern here is exactly what you should expect:

• number from 3–10
• followed by a plural noun

In fully vowelled MSA, this would be:

أنتظر أربعَ ساعاتٍ

Here is what is happening:

• أربعَ is accusative, because the whole phrase expresses duration after the verb.
• ساعاتٍ is genitive plural, because after numbers 3–10, the counted noun is typically plural and genitive.

So the case pattern is:

• أربعَ = accusative
• ساعاتٍ = genitive

In normal unvowelled writing, these endings are not shown, so you just see أربع ساعات.

A careful MSA pronunciation is:

ʾantaẓiru ʾarbaʿa sāʿātin

Very roughly for an English speaker:

• أنتظر ≈ an-ta-zi-ru with an emphatic ẓ
• أربع ≈ ar-ba-ʿa
• ساعات ≈ saa-ʿaat

The letter ع in أربع and ساعات is a consonant that does not exist in English, so that part usually takes practice.

By itself, أنتظر is normally understood as present or ongoing:

• I wait
• I am waiting

If you clearly want the future, Arabic usually adds a future marker such as:

• سأنتظر = I will wait
• سوف أنتظر = I will wait

So without a future marker, learners should usually read أنتظر as present/habitual unless context says otherwise.

Yes. This is a very normal Arabic order:

• أنتظر = verb
• أربع ساعات = duration expression

Arabic often begins sentences with the verb, so verb + time expression is perfectly natural.

You could add أنا at the beginning for emphasis:

• أنا أنتظر أربع ساعات

But the shorter version is completely natural.

Not always exactly.

أنتظر أربع ساعات gives the idea of waiting for four hours as a duration. But if you specifically want the English meaning I have been waiting for four hours (an action that started in the past and is still continuing now), Arabic very often uses منذ:

• أنتظر منذ أربع ساعات = I have been waiting for four hours

So your sentence is grammatically fine as a duration expression, but for the very specific have been waiting meaning, منذ is often more natural.

Because after a number like أربع, the counted noun is usually indefinite unless the context makes it definite in some special way.

So:

• أربع ساعات = four hours

not normally:

• الأربع ساعات

In ordinary counting expressions, Arabic usually leaves the noun without الـ.