I found this passage in Shanti Parva of Mahabharat:
Assuming a form that is manifest, I dwell, at present, in the heavens. At the end of a thousand Yugas I shall once more with-draw the universe into myself. Having withdrawn all creatures, mobile and immobile into myself, I shall exist all alone with knowledge only for my companion. After the lapse of ages I shall again create the universe, with the aid of that knowledge. That which is my fourth form creates the indestructible Sesha. That Sesha is called by the name of Sankarshana. Sankarshana creates Pradyumna. From Pradyumna I take birth myself as Aniruddha. I create (myself) repeatedly. From Aniruddha springs Brahman. The latter takes birth from Aniruddha's navel. From Brahman spring all creatures mobile and immobile.
The partial sanskrit verse of the same is:
asman-murtish caturthi ya sashrijac chesham avyayam sa hi sankarshanah proktah pradyumnam so 'py ajijanat pradyumnad aniruddho 'ham sargo mama punah punah aniruddhat tatha brahma tan-nabhi-kamalodbhavah brahmanah sarva-bhutani sthavarani carani ca
Observe the bold section in the above English passage.
Who exactly is that 'I' in the passage? What exactly is the meaning of 'That which is my fourth form creates...'? If Sesha is fourth form, what are the other three forms?