First off, the number would actually have to be five. Because you're using your array as a string, you need to include space for the terminating null character. Thus, you have to add one extra cell for the size.

Next, do you know how to use a loop? If not, you can do it without, but it's just more lines of code.

If you do, then simply access each element in the array using a loop. I'll show you the general idea without, and you can apply it to the 'with':
Code:
char swapper;
char array[5] = "1234";

swapper = array[0];
array[0] = array[3];
array[3] = swapper;
There, you have successfully swapped two digits in your string.

Quzah.