As explained earlier in the same chapter, the four forms are referring to Viṣṇu's catur-vyūha forms. 'I' refers to the inactive Puruṣa in the 24/25-form of Viṣṇu (ahaṃ hi puruṣo jñeyo niṣkriyaḥ pañcaviṃśakaḥ).
Chapter 1654 (326)
'Bhishma continued, "Having thus shown himself to Narada, the one
generated from Parameshthi again spoke these words. 'O Narada! Without any
delay, leave this spot. These devotees of mine are like the moon in their
complexions ... There is
no doubt that they will merge into me.
There is one who cannot be seen with the
eyes. He cannot be touched with touch. He cannot be smelt through smell. He is
beyond taste. He is beyond the qualities of sattva, rajas and tamas. He is a
witness to everything and people speak of him as the atman. He is in the body of
all creatures and is never destroyed. He is without birth. He is everlasting. He is
eternal. He is without qualities. He cannot be divided into components. He is
beyond the twenty-four principles and is known as the twenty-fifth. He is the
passive Purusha and is said to be comprehended through knowledge. O
supreme among brahmanas! A person who merges into him is emancipated. He
is known as Vasudeva. He is the eternal paramatman.
O Narada! Behold the
greatness and power of that god. He is never touched by good and bad deeds.
Sattva, rajas and tamas are said to be the qualities. These exist in all bodies and
roam around there. Though these qualities are enjoyed, kshetrajna does not
enjoy them. He is without qualities. He is free from the qualities. He is the
creator of qualities. He is superior to the qualities. O devarshi! When the
universe is destroyed, the earth merges into water. Water merges into light.
Light merges into the wind. The wind merges into space. Space merges into the
mind. The mind is a supreme element and it merges into the unmanifest. O
brahmana! The unmanifest merges into the inactive Purusha. There is nothing
that is superior to the eternal Purusha. There is no creation in the universe,
mobile or immobile, that is eternal. The only single exception is the eternal
Purusha Vasudeva. The immensely powerful Vasudeva is in the atmans of all
creatures. The great-souled one exists in bodies, known as the accumulation of
the earth, the wind, space, water and light as the fifth. O brahmana! Though he
cannot be seen, he uses his great valour to enter. That is how birth occurs,
through the efforts of the lord. Without a combination of the elements, there
cannot be a body. O brahmana! Without the jivatman, that combination of
elements wouldn't have moved. That jivatman is also known as the lord Shesha
or Samkarshana. Using his own deeds, the one who arises from this is known as
Sanatkumara. When all creatures are destroyed, it is into him that they merge.
The mind of all creatures is known as Pradyumna. It is from him that the the
doer and cause and effect arise. Everything in the universe, mobile and
immobile, is generated from Aniruddha. He is also known as Ishana and he
manifests himself through all his deeds. The illustrious Vasudeva is kshetrajna
and possesses no qualities in his soul. When that illustrious one is born in any
creature, he is known as the lord Samkarshana. Pradyumna is said to be
generated from the mind and is born from Samkarshana. Aniruddha is
generated from Pradyumna and is consciousness, or Maheshvara. Everything in
the universe, mobile and immobile, is generated from me. O Narada! This is
true of the destructible and the indestructible, the existent and the non-existent.
Those who are devoted to me, enter me and are emancipated. Know that I am
the inactive Purusha, the twenty-fifth. I am without qualities. I am without
separate constituents. I am without opposite sentiments and I am without
possessions. You will not understand this, since you are seeing me in a form. However, if I so desire, I can make this form disappear instantly. I am the
preceptor of the universe. O Narada! That you can see me is only because of a
maya that has been created by me. I seem to possess the qualities of all
creatures, but that's because you cannot comprehend me. I have appropriately
told you about my four forms.
. . .
At the end of one thousand Yugas, I will destroy the universe again. I will withdraw all mobile and immobile objects into myself. O supreme among brahmanas! I will then exist alone, with nothing except knowledge. Using that knowledge, I will again create everything in the universe. My fourth form creates the undecaying Shesha. This is also spoken of as Samkarshana, and Pradyumna is created from this. From Pradyumna, I repeatedly create myself as Aniruddha.
Brahma, the one who is born from the lotus, results from Aniruddha. All mobile and immobile creatures are created by Brahma. Know that this repeatedly happens in several kalpas . . .
[The Mahabharata: Volume 9, Bibek Debroy]