#include <stdio.h>
#include <stdlib.h>

main ()
{
   int number = 0, a1 = 0, a2 = 0, a3 = 0, a4 = 0, a10 = 0;

   printf("Please input char 4 ");
   scanf("%d", &number);

   a1 = number / 1000;
   a2 = (number - (a1 * 1000)) / 100;
   a3 = (number - ((a1 * 1000) + (a2 * 100))) / 10;
   a4 = (number - ((a1 * 1000) + (a2 * 100) + (a3 * 10)));

   a10 = a1 * 8 + a2 * 4 + a3 * 2 + a4 * 1;

   printf("%d", a10);
   system("PAUSE");
}

int c, sum = 0;
while ((c = getchar()) != EOF) {
    if (!(c == '0' || c == '1')
       break;
    sum = sum * 2 + (c-'0');
}
printf ("%dn",sum);

nextDigit = currNumber % 10;
currNumber = currNumber / 10;

currNumber = currNumber * 8 + nextDigit;

int nextDigit = inputNumber % 10;
inputNumber = inputNumber / 10;
targetNumber = targetNumber * 8 + nextDigit;

a10 = 0;
factor = 1;
while(number) {
   a10 += number%10 *factor;
   number /= 10;
   factor *= 2; // или factor <<= 1;
}