I think I know why: you're reading them in in a way that you did not expect. 1234 is not 4 integers, but one integer.
I already tried both with and without address.. I knew that & will show the address and not the value, and I tried both with and without & yesterday before wiretron told me here =P
I suggest you change to use chars instead of ints, e.g.,
char a, b, c, d;
printf("Enter 4 digits: ");
scanf("%c%c%c%c", &a, &b, &c, &d);
printf("In reverse: %c%c%c%c", d, c, b, a);