There is no evidence at all of a fourth child, though your Battlespire/Divayth Fyr idea holds some water. This theory has also been conceived by other forum members, too.
This said, no one has found enough evidence to sufficiently substantiate this idea.
I find the idea far-fetched. Divyath's "daughters" are more like clones. Therefore, if there was another 'child' it would be Dunmer in appearance.
I highly doubt the hero of Battlespire was a Dunmer, given that the player character in Battlespire is supposedly training to be a Battlemage (?). :shrug: