Prepare a work surface (kitchen counter) by laying out a 4 feet long piece of parchment paper.
After popping the popcorn, transfer it to a bowl. Lift the popcorn with your hands and shake the bowl lightly so the unpopped kernels fall to the bottom. Lift the 4 quarts of popcorn out of the bowl and transfer to a large brown paper grocery bag.
In a medium-size non-stick sauté pan, bring the apple cider to a boil over medium high heat. Boil vigorously, stirring occasionally, for 8-10 minutes or until the cider is reduced to ¼ cup. Transfer the cider to a large microwave-safe bowl.
Add the corn syrup and stir well to combine. Add the butter, brown sugars, cinnamon, nutmeg and salt and stir until everything is well incorporated. Place in the microwave and heat on high power for 3 minutes.
After stirring, return to the microwave for 1 minute. The mixture should be bubbling up furiously by this point. Remove bowl and stir well then return to the microwave for another minute. Repeat this cooking and stirring for a total of 3 minutes.
Remove the bowl from the microwave and stir in the vanilla and baking soda. Stir until all of the baking soda is well incorporated. Pour the hot mixture over the popcorn in the bag. Fold the top of the bag down once and shake, shake, shake to distribute syrup through the corn.
Microwave on high power for 1 minute. Remove the bag and hold it at the folden down top and one of the bottom corners. Shake well the return to the microwave for another minute. Repeat cooking and shaking two more times for a total of 5 minutes. Turn the popcorn out onto the prepared parchment paper. Allow to cool completely.
Store in an airtight container. The caramel corn will keep well for 1-2 weeks.