Based on Sri P.B. Annangaracharyar Swami's Commentary

Shloka 4:

	trayyAmA~NgalyasUtram triyugapadayugArohaNAlambasUtram
		sadvidyA dIpasUtram sakalakalikathA samhR^ite: kAlasUtram |
	pra~nAsUtram budhAnAm prashamadhanamana: padminI nAlasUtram
		rakshAsUtram munInAm jayati yatipatervakshasi brahmasUtram ||

Meaning:

trayyA mA~NgalyasUtram		- The thread that is like the holy matrimonial thread 
				  around the neck of mother Veda;
ArohaNa AlambasUtram		- the string that helps climb to 
triyuga padayuga		- the divine feet of the Lord;
sadvidyA dIpasUtram		- the thread that helps the lamp that is Upanishad, glow;
sakalakalikathA samhR^ite: 
kAlasUtram			- the thread (like the one Yama carries) 
				  that destroys the activities of Kali yuga;
budhAnAm pra~nAsUtram		- the thread that helps expand the minds of devotees;
prashamadhanamana: padminI 
nAlasUtram			- the thread that is like the stalk of the lotus which 
				  is the heart of the panditas whose wealth is knowledge;
munInAm rakshAsUtram		- the thread that protects such panditas;
brahmasUtram yatipate: 
vakshasi jayati			- such a Brahmasutram of Ramanuja shines on his divine chest.

Commentary:
Similar to the previous two shlokas where Ramanuja's tridandam was praised, in this shloka, the holy thread on the chest of Ramanuja is enjoyed and through it the greatness of Ramanuja is brought out.

In other sampradhayams, when a person enters the sanyasAshrama dharma, they remove their hair completely and throw away their holy thread. In Srivaishnava sampradhaya, the sanyasis keep a tuft of hair on their head and continue to wear the holy thread. Thus, in this shloka the beauty and qualities of the holy thread worn by Ramanuja is being appreciated.

trayyA mA~NgalyasUtram:
The holy thread on Ramanuja's divine chest is like the matrimonial thread worn around the necks of women to show that they are married. This description is the same that Sri Parasara Bhattar uses to describe the works of Swami Kooraththazhvan in his thaniyan ("yaduktayas trayIkaNThe yAnti mangaLasUtratAm").

If a woman is not wearing the matrimonial thread around her neck, either she is still unmarried or she is a widow. If it is the former, many men will think of themselves as her possible husband; in the case of the latter, people would know that she has lost her husband. Similarly, prior to Ramanuja's birth, people thought of many devatas as the Supreme (as the paramapurusha of the Vedas) while there were others who claimed that there is no Supreme (as described in the Vedas). After Swami Ramanuja's birth, it was made clear to one and all that there is a Supreme and that He is Sriman Narayana - thus the matrimonial thread shone around the neck of the Veda mAta.

triyuga padayuga:
yuga means a pair. tri yuga is therefore six - this refers to the six divine qualities (knowledge, power, strength, wealth, valor and splendor) of the Lord as shown by the word Bhagavan. Therefore, triyuga refers to the Lord. pada yuga refers to the divine feet of the Lord.

ArohaNa AlambasUtram:
When climbing a mountain, one uses a rope to pull themselves up. Similar to that, Swami Ramanuja's thread helps devotees make the ultimate climb - to the divine feet of the Lord. Vedanta Desikan uses another comparison for this idea - Ramanuja is like a lion jumping between mountains (this world and the next); and the devotees are like small insects that would never get across these mountains in many lifetimes; but the insects that cling on to the lion will reach the other mountain along with the lion in one bound.

In some texts for this shloka, the phrase used is triyugayuga. In that case, the number shown is 12 (3 x 2 x 2). This numbers refers to the archirAdhi path. Then, it is shown that Swami's thread helps the souls go through this path. That is, for those who have surrendered at Swami's feet, the path of archirAdhi has become easily attainable.

sadvidyA dIpasUtram:
Without a thread (wick), a lamp would not glow. Thus, without Ramanuja's thread, the Upanishads that are lamps would not glow.

sakalakalikathA samhR^ite: kAlasUtram:
The Lord of death, Yama, carries in his hands a rope called kAlapAsha, that he uses to capture souls from their bodies. Similar to that, Ramanuja's thread destroys the uproar caused by the activites of Kali yuga.

budhAnAm pra~nAsUtram:
Ramanuja's thread brightens and expands the thoughts of those that surrender at his divine feet.

prashamadhanamana: padminI nAlasUtram:
Yogis are those that have highest qualities as their only wealth. Such yogis were Ramanuja's disciples such as Embar, Arulala Perumal Emberumanar, et al. Their hearts are like lotuses that blossom in the presence of Sun who is Ramanuja ("rAmAnuja divAkara:"). To their lotus minds, Ramanuja's thread acts as the stalk - connecting them to Ramanuja and nourishing them.

The Sun that is Ramanuja will both drive away the darkness caused by other sampradhayams ("nirAsa vAsarapati:") as well as blossom the hearts of the true devotees.

munInAm rakshAsUtram:
The thread also protects the same yogis.

vakshasi jayati:
Unlike sanyasis of other sampradhayams, Ramanuja wears the Brahmasutram. In this shloka, it is said that even the act of enjoying the beauty of that sacred thread will provide refuge and achieve liberation.