4 month old not able to stay upright

Is it hunched or puffed up (the head being pulled in?) Or is the neck bent down or to the side? Hunched posture with the neck pulled in is a typical sign of a sick or cold chicken, sometimes from coccidiosis. Cocci symptoms are lethargy, not eating, diarrhea or blood in droppings, and puffed up posture. Loss of balance or falling over, laying around can be a symptom of Mareks disease, or from being very weak or dehydrated (again sometimes from cocci or other problems.) I would get some Corid powder or liquid, and start a 5 day treatment in the water, for possible coccidiosis. Until you get that try to get the sick chick to drink water and eat a little plain yogurt or some egg. After a 5 day course of Corid, treat with some vitamins and probiotics for a few day. Lastly, I would worm all of the birds with Valbazen 1/2 ml orally, and repeat it in 10 days. If you can't find Valbazen, you can order it online, or get some SafeGuard liquid goat wormer. Here are a couple of links about coccidiosis:
http://www.chickenvet.co.uk/health-and-common-diseases/coccidiosis/index.aspx
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html


puffed up chicken wry neck or torticolis, sometimes from Mareks disease