Guess the number magic

Let the subjects drill a positive integer x, including 0, and let the subjects remember what they chose. Forget it, then multiply X by 2 to get A, then add A to 8 to get B, then divide B by 2 to get C, and subtract the first selected X number from C to get 4. No matter what X you choose, the final answer is 4. Then you can add something casually at the back, and finally tell the answer mysteriously.