Air pop or microwave kernels in a paper sack (do ¼ cup at a time to prevent burning, re-using same paper sack) or air popper. Pick out the un-popped kernels. Put freshly popped corn in a LARGE bowl (or two smaller bowls) with extra room to stir in caramel later.
Combine sweetened condensed milk, brown sugar, corn syrup & butter in a saucepan. Let boil for 2½ minutes stirring constantly. Pour over popcorn. Stir until coated (BE CAREFUL NOT TO BURN YOURSELF WITH THE HOT CARAMEL!). Enjoy warm! Store any leftovers in a ziplock bag.