Try:
printf("%04x",a);
0
- Left-pads the number with
zeroes (0) instead of spaces, where
padding is specified.4
(width) - Minimum number of
characters to be printed. If the
value to be printed is shorter than
this number, the result is right justified
within this width by padding on the left
with the pad character. By default this is
a blank space, but the leading zero we used
specifies a zero as the pad char.
The value is not truncated even if the result is
larger.x
- Specifier for hexadecimal
integer.More here