Technically there are a few more Great Bastards (Spoilers Extended)

In ASOIAF, the bastard sons and daughters born to Aegon IV from noblewomen were known as Great Bastards. They include the following:

• Daemon Blackfyre (Waters)

• Aegor "Bittersteel" Rivers

• Mya Rivers

• Gwenys Rivers

• Brynden "Bloodraven" Rivers

• Shiera Seastar

He had numerous other children and was even rumored to be the father of Viserys Plumm, but if we look at another event, we see that its probable that he impregnates at least 3 other noblewomen in his life:

Aegon soon filled his court with men chosen not for their nobility, honesty, or wisdom, but for their ability to amuse and flatter him. And the women of his court were largely those who did the same, letting him slake his lusts upon their bodies. On a whim, he often took from one noble house to give to another, as he did when he casually appropriated the great hills called the Teats from the Brackens and gifted them to the Blackwoods. For the sake of his desires, he gave away priceless treasures, as he did when he granted his Hand, Lord Butterwell, a dragon's egg in return for access to all three of his daughters. He deprived men of their rightful inheritance when he desired their wealth, as rumors claim he did following the death of Lord Plumm upon his wedding day. -TWOIAF, The Targaryen Kings: Aegon IV

and:

"King Aegon presented the egg to his father's father after guesting for a night at his old castle," said Ser Maynard Plumm.

"Was it a reward for some act of valor?" asked Dunk.

Ser Kyle chuckled. "Some might call it that. Supposedly old Lord Butterwell had three young maiden daughters when His Grace came calling. By morning, all three had royal bastards in their little bellies. A hot night's work, that was." -The Mystery Knight

If Aegon IV impregnated the three noblewomen of House Butterwell then he has 3 more Great Bastards. Keep in mind that these are maidens, so the acknowledgement is forced:

"There's proof of a sort at Storm's End. Robert's bastard. The one he fathered on my wedding night, in the very bed they'd made up for me and my bride. Delena was a Florent, and a maiden when he took her, so Robert acknowledged the babe. Edric Storm, they call him. He is said to be the very image of my brother. If men were to see him, and then look again at Joffrey and Tommen, they could not help but wonder, I would think." -ACOK, Davos I

So using this logic, if any of Lord Butterwell's daughters gave birth, Aegon IV would have been forced to acknowledge the children.

There are some possible options as to why we have never heard of any of these children wrt to the series:

• Author error: This was meant to be world building and wasn't remembered when GRRM created the "Great Bastards"

• Kyle the Cat being wrong: TWOIAF only says "access to 3 daughters", Kyle is where we get the info about the bastards and that could just be a rumor

• Death: 1 or more of the children could have died while still in the womb, birthing or early in life

• Intentional: As having Targaryen blood, they have intentionally been omitted from the story so far and could come back into play later

TLDR: Due to Aegon's IV's lusts, he traded away a dragon egg and potentially added 3 more Great Bastards to the story