switching to formula

I've been breastfeeding without any problems for 5 months and now i have 2 jobs and one of them is only 4 hours and even with just that one job its hard to keep up with pumping enough for my daughter when she cluster feeds constantly whenever im around, i really dont want to switch over but i need her to have enough milk to keep her full. I also have started feeding her organic rice cereal trying to avoid formula but i dont know what to do. any advice?