Post Holdings will buy Lakeville-based MOM Brands for $1.15 billion, combining the nation’s third and fourth largest breakfast cereal makers. The deal, announced Monday morning, will give publicly traded Post a presence in the growing bagged cereal and hot cereal businesses, two of MOM Brands’ strongholds. Privately-owned MOM Brands, which has a big cereal plant in Northfield, is known for its Malt-O-Meal brand. The 95-year-old company has grown steadily over the past 15 years, particularly capturing share in the low-price or “value” segment of the cold cereal business.