S.K.
12 is the most often recommended age to be left alone for a few hours, only a few states like Illinois and Maryland have a set minimum legal age. Everywhere else you have no protection - it is judged on an individual basis. And given you have 4 kids and they range from newborn to 9 (as the oldest!!)if someone sees you leave your kids in the car and they call the cops, cps is going to be called in.
Something else I would consider - you're leaving your 9 yr "in charge" of the other kids - if something actually happened and one or more of the younger ones were taken because your 9 yr old CANNOT overpower an adult, how is she going to live with that the rest of her life? Please don't do that to your children. Don't put their lives at risk like that and don't make your daughter assume responsibilities that she cannot possibly fulfill adequately.