packing and unpacking bits

I've been reading on how you can compress the actual bits in a program. Below is my code. I've pieced it together, but all that I'm getting is the full 8 bits. My output reads like:

abcd= 01100001 01100010 01100011 01100100

and I was wanting/hoping it would show just the last 4 bits. Am I way off? What about if I was wanting to try numbers instead of letters? I just don't want to be barking up the wrong tree totally ya know? Don't laugh please. Anyway, below is my code for the output above. Any help or suggestions would be greatly appreciated. thanks!

#include <limits.h>
#include <stdio.h>

void bit_print(int a)
{
int i;
int n = sizeof(int) * CHAR_BIT;
int mask = 1 << (n-1);
for (i=1; i<=n; ++i)
{
putchar(((a&mask)==0) ? '0':'1');
a <<=1;
if (i % CHAR_BIT==0 && i < n)
putchar(' ');
}
}

int pack(char a, char b, char c, char d)
{
int p = a;
p = (p<<CHAR_BIT)|b;
p = (p<<CHAR_BIT)|c;
p = (p<<CHAR_BIT)|d;
return p;
}

int main()
{
printf("abcd= ");
bit_print(pack('a','b','c','d'));
putchar("\n");
}