It doesn't sound like it's going to be edible. I mean a chicken forgotten about at 400 degrees for several hours is going to be either totally desiccated or reduced to charcoal.
Of course, there won't be any bacteria - 400 degrees for several hours will kill everything - and a few hours is more than enough for the heat to soak all the way through the chicken to bring the whole thing up to sterilization temperature. Although the chicken would have been riddled with dangerous bacteria prior to cooking - you can be fairly confident that you have left no survivors.
6 hours after cooking, is unlikely to be enough time for dangerous bacteria to appear from the air and leave a toxic mess for you - particularly, if the oven has been cooling naturally during that time.
On the other hand, doggie bag food is slightly different. Once served, there is the potential for bacteria to land on it, and settle and breed. Some bacteria (particularly the opportunistic ones like Staphylococcus) produce toxins as they grow. Although, when you nuke them prior to eating, you sterilize the bacteria - you don't do anything about the toxins. This means that re-cooking food is not a satisfactory method of avoiding food poisoning - and appropriate storage following (and prior to) cooking is required.