You fill up the 3 gallon container and pour the 3 gallons into the 5 gallon container. You fill the 3 gallon container back up, and pour it into the 5 gallon container. You empty the 5 gallon container and pour the 1 gallon left in the 3 gallon container into the five gallon container. Then you fill the 3 gallon container back up and pour it into the 5 gallon container and you have 4 gallons.
One thing Julie could do is freeze a tray of ice-cubes, and turn the tray of ice upside down in her freezer. When she comes home, she should check the tray. If the ice cubes are still in the tray, the food is safe to eat. If the trays are empty, it's time to clean out the freezer. She will have to make a judgment call if the ice-cubes are only slightly thawed.