In plain words
Śatrughna, who bore… on his head the commands of these three, Rāma, Lakṣmaṇa and Bharata, destroyed the enemies within and without and brought peace, is the Atharvaṇa Veda itself. So Rāma, Lakṣmaṇa, Bharata and Śatrughna are the very form of the Yaju… Veda, the Ṛgveda, the Sāmaveda and the Atharvaṇa Veda. When we look with … tattva, the Rāmāyaṇa teaches us a fine meaning.
Not only this: Oṃkāra is spread throughout the śāstras. ‘Om ityekākṣaraṃ brahma’: the Brahman-tattva… that is one syllable is Oṃkāra. Oṃkāra is the joining of A, M and U. A is Lakṣmaṇa, U is Bharata…, M is Śatrughna. The joining of these three letters, Oṃkāra, is Rāma. The śāstras have made it clear that the Rāma-tattva is Oṃkā…tattva. Moreover, Vālmīki …ed the nature of Rāma, Lakṣmaṇa and Sītā as Oṃkāra. Lakṣmaṇa, who is A, stays on the right; Rāma, who is U, stays in the middle; Sītā, who is M…, stays on Rāma's left side. The very form of this A, U and M is … Rāma and Sītā. Vālmīki went on describ…ing the oneness of these three as the Ātmatattva joined with divinity; … where are Rāma, Lakṣmaṇa, Bharata and Śatrughna, or any other great powers? Are they …, or are they within? Who is the father of these four sons? He is Daśaratha. Which kingdom did this Daśa… … go on ruling? It is Ayodhyā. How many ramparts has Ayodhyā? Seven ramparts. A city of … miles? It is twelve yojanas round. What is the inner meaning of this? Ayodhyā, Daśaratha, Rāma: all are contained in man himself. Ayodhyā means a place that warriors … reach…. That is the heart. Who rules it? Mahārāja Daśaratha rules it. … Daśaratha? Five organs of action, five organs of knowing: the body with these ten senses is itself Daśa…. Ayodhyā has an extent of twelve yojanas. The Paramātma has the name Saṃvatsara. A year has twelve months. Within these twelve months there are twelve new moons and twelve full moons; …, new moon, joy and sorrow: this heart is joined with both of these. This heart is … with pleasure and pain. This body bears the three guṇas. This chariot called Daśaratha wedded the … and tamo guṇas. The sattva guṇa itself is Kausalyā. The rajo guṇa itself is Sumitrā. Kaika is the tamo guṇa. … wedded these three guṇas as his wives. For the body that has these three guṇas, what is …? The four puruṣārthas are the chief thing. Dharma, Artha, Kāma and Mokṣa are the right paths for it. These are the …. These puruṣārthas themselves were born as Rāma, Lakṣmaṇa, Bharata and Śatrughna.
So, to make the puruṣārthas worthwhile, these four kinds of Dharma … came into being.