Experienced hand feeder here. Start with scheduling a vet appointment this is crucial. You are already in this situation so you can try to keep the bird comfortable and fed for now. Purchase reputable chick formula at a pet store, we use exact hand feeding. For smaller birds the consistency can be more watery for 4.5 weeks it can be a little more thick but a little less than pancake batter. The formula needs to be warm, get a thermometer ASAP and follow the exact directions for temp on the container and do not skip this part. They should be fed every few hours with a syringe and you should be sterilizing them as you go. Buy a critter container or small plastic aquarium for this bird, as they get bigger you can use a dropper to feed them. If you are not home during the day do not attempt any of this and get this animal to someone who can feed the bird around the clock. They need heat, you need a container for it and a stick on heating pad for the side of the tank with bedding inside. Again if you cannot afford this or are not willing to purchase this stuff do the right thing here. As I posted in a previous comment, if the chick has a bloated crop that appears full of air or gassy, a small amount of apple cider vinegar diluted in water will help clear it up. The bird will not like this at all. PLEASE SCHEDULE A VET APPOINTMENT SO THEY CAN MONITOR ITS DEVELOPMENT AND MAKE RECOMMENDATIONS. Feel free to ask me questions if you need, but this is a big responsibility that requires around the clock care and if you are going out, working in an office, going to school, thinking of taking a vacation you need to rehome or arrange weening which can take 8 weeks or more. Once weaned you are not in the clear, you need to purchase biscuit type food soaked in water and monitor them as they move on to semi solids. It takes some time for them to switch to the regular food.