@Kiki do you have info for Vets or Labs in Houston?
Do your birds have shade?
You mention her crop is "normal", when are you checking her crop - is it emptying overnight?
The poop looks muddy looking. Are you spraying down or adding water to your run to try to keep the birds cool?
If you are going to treat with Corid, then stop all vitamins/supplements until you finish the course of Corid treatment.
Liquid Corid (9.6%) dose is 2 teaspoons per gallon of water given as the only source of drinking water. If you feel that Coccidiosis is the issue of the ones you lost as well, then treat all your chicks.
For this one, if you feel that Coccidiosis is the problem, then direct oral drench her with undiluted Liquid Corid at a rate of 0.10ml per pound of weight once a day for 3 days.
This is in addition to you syringing or her drinking as much of the mixed Corid water that she should consume (normally drink) in a day.
You want her hydrated.
Since you have lost 3 already, it would be a very good idea to have a necropsy performed if you lose this little one too. This way you will know more about the cause and what steps/treatments you may need to take.