Hmm. I don't know. Set the furnace at 45 degrees and I would be surprised if the batteries didn't last 4 days. Batteries don't last long with furnaces when trying to keep it at 70 degrees but set the temp down to 45 and I think it would probably make it. Warm the unit up to about 80 degrees before you leave and then set the temp to 45. The furnace won't even come on for the first 10 12 hours. I had to do something like this with our little C once. All went well. Plug it into the truck when pre-heating it. Or just winterize it. :)
JMHO.