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.

No tiny little flowers. (I'll try to get a better pic as well.) I live in VA, grass is TTTF with some KBG. I used Bayer Advanced Lawn Week Killer.

One of the reasons I think this stuff came up is I did not use a preM this fall because I aerated and overseeded in September. The yard looked astounding, then this stuff started coming up.

One of the arguments against aerating is that it brings weed seeds to the surface.

If you can get a picture from about 3 inches away that will help. If it has flowers, that helps, too.

Looks like chickweed to me. Ortho CCO should take it out. The hot weather in summer will do it in too. I don't know about using herbicides in February. The recommended time is when the weed is actively growing.So it might be better to wait until April. A pre-emergent in the fall will help prevent next year's seeds from germinating.

It's Chickweed. In mild climates it will flower and seed all winter. It needs clear soil to germinate and is usually a weed of cultivation rather than lawns. So if you can thicken up the sward it shouldn't be able to dominate.

Call Ortho customer service and ask about applying Weed B Gon CCO in the winter. I've now read something that says it's better to tackle weeds early and to do it before the spring greenup. Ortho should know if that is recommended for its product. You can also just get out and pull it up. It pulls up easily. A single plant takes up a lot of area. You may be surprised at how much you can get up by pulling.