It's probably lacto. When you threw in the grain, you added lacto and anything else that was on the grain (e.g. other bacteria, mold spores, other fungus, etc.) and during those four days all of those critters had time to build up a little colony. Although the lacto and later yeast may have won and fermented the fermentable sugars, the other critters you introduced may have had the opportunity to rise up and start consuming unfermentable sugars.