Wilson Industries sources from multiple suppliers and is considering the aggregation of inbound shipments to lower its costs. Truckload shipping costs $$500$ per truck along with $$100$ per pickup. Average annual demand from each supplier is $10,000$ units. Each unit costs $$50$ and Wilson incurs a holding cost of $20\%.$ What is the optimal order frequency and order lot size if Wilson decides to aggregate $4$ suppliers per truck?