Why did Bhagavan take avataras as the four sons of Dasharatha?
Why not just one?
Just Rama.
Why Lakshmana, Bharata and Shatrughna also?

Let us see this today.

Rishyasringa is conducting Putreshti for Dasharatha.
Ashwamedha yaga is completed.
Dasharatha's all past sins have been washed away.
Now Putreshti should make him achieve his desire.
Dasharatha wants successors.

In the Putreshti all Devas have been invoked as per the procedure given in Atharva Shiras.
They complained to Brahma about the atrocities of Ravana.
Brahma said Ravana can be eliminated only by a human.

As this conversion is going on Bhagavan Mahavishnu appears there in all his glory.
Everyone bows to him and pray to him for relief from Ravana.

Bhagavan's visit has two purposes.

One- To bless Dasharatha with whatever he wants.
Two - To take care of this problem.

Bhagavan must have thought why not combine both.

But, Devas are praying that you please take birth as four sons of Dasharatha, in three of his wives.

तस्य भार्यासु तिसृषु ह्रीश्रीकीर्त्युपमासु च।
विष्णो पुत्रत्वमागच्छा कृत्वात्मानं चतुर्विधम्॥

Dasharatha's three wives are modesty, prosperity and fame embodied.

Why four sons?
It is a specific request.
Is Ravana so strong that the Lord can not manage him alone?

No.
It is not just Ravana.

चातुर्विध्यविधानं रावणरावणिलवणगन्धर्वविनाशाय।

Ravana, his son, Lavnasura and the troublesome Gandharvas.
All four have to be eliminated.
Each task one of the sons will perform.
There are specific requirements in each case.
Such as only someone who has observed brahmacharya and not slept for 12 years can kill Indrajit.
That Rama will not be able to do.
He has Devi Sita with him.

All details are planned.

Taking birth as four sons at the same time easy for Bhagavan.
Because he always exists and Chaturvyuha.
Sankarshana, Vasudeva, Pradyumna and Aniruddha.

Ravana has the power of Brahma's boon that he can be killed only by a human.
Ravana has his own muscle power as well.
He is cruel.
He kills Rishis just for fun.
He can not be killed by us.
We, Devas have come to you along with Munis and Siddhas.
Only you can save us.

Bhagavan said: stop fearing.
Devas fear is not the usual fear about the power of Ravana.
Devas fear is due to the fact that their hands are tied.
They can't touch Ravana, because Brahma has given him boon.
That they will not harm you.
Devas can not offend Brahma by violating his boon.

Bhagavan said: stop fearing.
I will take care of it.
I will take eliminate Ravana along with his entire clan.
Not only that.
Dharma is disturbed now.
The divine order that I had set for administering the world is disturbed now.
Because of Ravana.
I will stay back and rule for eleven thousand years, ensure that divine order is re established, then only I will return to Vaikuntha.

This is how Bhagavan outlined his plan for Ramavatara.

Why avatara as human is clear now.
Why Dasharatha and his wives should be the parents is clear now.
Why as four sons is clear now.
Why Ravana needs to be eliminated is clear now.
Why the Lord ruled for eleven thousand years is also clear now.