Sutra:
49. chathurththi ERinapadi en? ennil.

50. nArAyaNa pathaththukku sangkrahamAy irukkaiyAlE.

Meaning:
49. While there are sevaral cases (vibhakti, vERRumai) present, why whould one take it that it is the fourth case that is present inside the akAram?

50. Because, the akAram is the essence of the nArAyaNAya that follows in the Thirumantra. And in that term, the fourth case is explicitly present.

Sri PBA Swami's Sarartha Deepikai:
As there several cases in grammar, why should it be said that it is the fourth case that is present in the akAram?

It is because the akAram is the essence of the Narayanaya term that follows and Narayanaya is the expansion for the akAram. And in the Narayanaya term, it is the fourth case (chaturthI vibhakti) that is explicitly present. Therefore, it is correct to take it that the fourth case is present in the akAram.