Hard to tell from this altitude and the lens Vaseline factor. Are those tiny yellow flowers? If so I would suggest it might be horse herb. That can be tough to get rid of or easy depending on your lawn type. Where do you live? What kind of grass do you have? Can you be more specific about the brand and name of the 2-4,d product you used? They're not all the same. Furthermore if you have St Augustine, you may have killed the grass with that spray.

bumping to see if anyone has any ideas

Could be.... second photo is from too far a distance to be positive. Most broadleaved lawn weed killers will address this. Or you can always manually remove.

It would have had flowers at the top if it had lived longer. So lets call it Lactuca serriola, aka Prickly Lettuce.