how frustrating that it opened after 4 days...sounds like you are doing all the right things - pressure, clean, neosporin, bandage, and the flour/cornstarch sounds great, too. i have bandaged hundreds/thousands of paws as a vet tech but not many bird feet. some things that help with dog/cat paw bandaging:
-we would wrap in gauze up to the ankle or even a little higher (even if wound is on the bottom of the paw), then bandage tape over the gauze, and extend the tape a little beyond (1/4") the gauze so that 1/4" of tape was stuck to the fur. Then they couldn't fling it off. Could do the same thing with the chicken foot, put some tape directly on the skin. if hard to remove later, use rubbing alcohol if you can keep it out of the wound, if not, warm soapy water will loosen it.
-has anyone every tried an elizabethan collar on chickens??? hmmm...I'd be tempted to give that a try if they are pecking at the bandage.
another thought - since she is weak, maybe you could put her in a sling to keep her from getting the bandage off? I saw a pick on here from someone who created a mini-hammock for their chicken to help it recover from something without getting poop all over. the hen sat in the hammock with her 2 feet poked through.
do you have any nurse/farmer/vet tech friends who could give her stitches? would surgical tape or super glue help keep it closed? please check with others on this, I haven't tried it but have heard of it.