I would see if there are any neighborhood teens willing to work as a Mother's Helper for a couple hours while they are home from school. Someone 12 or 13, not yet babysitting but interested in getting some experience. Offer a decent hourly rate for just 2 hours at least a couple days, from say 3-5 PM or 4-6 PM, to pay with one or the other of the younger kids, especially, and give you a little bit of a break.
They can play board games, do dress up, dolls, whatever one of your kids likes to do; they don't have to occupy all the kids.