Rama, Lakshmana, Bharata and Shatrughna:
Why four sons to King Dasaratha?

[Source:  bhakti-list@yahoo.com  Digest Number 414
“Sree RamaKrishna Prabha” by Sri Tenkatchi RaamaNaathan in Telugu
translated by Sandhya Chandarlapaty <g_eggs@yahoo.com>]

A learned scholar who used to give religious discourses, once posed the question: "Do you know why four sons were born to Dasaratha?" and answered it himself thus:

"In reality, Dasaratha wanted only one son to be the heir to his throne and rule the kingdom after him wisely. The Devataas too prayed to Lord Vishnu for one Raama to destroy Raavana and relieve them of their sufferings. So, why was Dasaratha blessed with four sons on receiving the gift of the divine Paayasam after the Putra Kaameshti yaagam?”

Let us see the explanation given for this in ancient literature:

There are four ways in which Dharma is practised in this world. Lord Vishnu incarnated as four brothers in order not only to destroy adharma, but also to demonstrate to the people these four ways of practising  Dharma. 

The first one is ordinary Dharma (Saamaanya Dharma).
How should children behave towards their parents? How should a disciple conduct oneself with a guru? How should a husband and wife be towards each other? How should a king rule over his subjects? Answers to questions like these constitute Saamaanya Dharma.

This was demostrated by the Lord in His Avataar as Sri Rama. When Saamaanya Dharma is followed in this manner, one attains to a state where God's proximity means everything that is permanent. 

One comes to realize that except for this nothing else is permanent. That is why Sri Kulasekhara Azhvaar rejected the chance to rule even Indraloka, saying that God's proximity was enough for him. The author of  Sree Krishna Leela Tharangini wrote: " Krishna! All I want is to be buzzing around your lotus Feet like a bee."  The rapturous feeling of Bhakti for the Lord and leading one's life in the service of the Lord is called Sesha Dharma. Lakhsmana's life is an excellent illustration of Sesha Dharma.

The third kind of practice is called Vishesha Dharma -
constantly thinking about God even while living far away from Him. This is more difficult to practise than Sesha Dharma. It may not be difficult to live in God's proximity and constantly think about Him and serve Him. But living away from Him, constantly thinking about Him and leading a life completely dedicated to him is difficult. The great Bharata demonstrated through his life this difficult way of practising Dharma.

The fourth Dharma is extraordinary. It involves serving those who serve God. Shatrughna, who constantly served Bharata,  ideally illustrates the practise of Dharma in this manner. The Bhaava of the practioner of this Dharma is similar to that of the Sri Vaishnava saying: “Adiyen Daasaanudaasan." This is best illustrated in Mukunda Mala (Verse 27) where Sri Kulasekhara Aazhvaar prays for the boon of being a servant of the servant who serves the servant of the servant-attendant of the servant of SrIman-nArAyaNa: 
bhRtyasya bhRtya iti mAm smara lOka-nAtha

Well, now we all know why Dasaratha was given four sons though he performed the Putrakameshti yagnam for the purpose of getting one heir to his throne! 

(NOTE: I have taken the liberty of editing the text slightly to convey the core idea according to my understanding of the subject.)