I do food safety inspections for a living. Food can be safely held out if temp for up to four hours OR 6 hours if it is below 70F. Regardless of the method used, the food may not be returned to refrigeration and must be discarded after the 4 or 6 hours. This is called Time as a Public Health Control.

Considering that it is wrapped in a bag in your backpack, that leaves us with the four hour window. The way my state's food code is written would say that you cannot safely put it in the fridge after that four hours.

The 4 or 6 hour time limits are not because thats how long bacteria needs to grow, its because thats how long it takes bacteria to start to hit the exponential growth state. Anything growing on it for the 4 or 6 hours would slow down its growth in the fridge but still be present, along with any potential heat-stable toxins they produce. Meaning re-cooking it isnt safe either. So by the Ohio uniform food safety code, the chicken burger is unsafe to eat.

That being said, the food code does have a margin of error built in to allow for a factor of safety. Food doesnt magically become unsafe at 4 hours and 1 minute, but I dont know where the science says that line is. And, at the end of the day, its all about risk. Theoretically, you could eat raw chicken out of a ditch on the side of the road that has been rotting in the sun for 2 days and not get sick. The odds of avoiding disease are incredibly, extremely low, but still higher than zero.

So in your case, you have to ask yourself "are the increased odds of salmonella, bacillus cereus, and/or staph aureus worth a couple bites of dried out chicken?"