The source code I found for strlen function in c is the one in below:

int
strlen(string)
    char *string;       /* String whose length is wanted. */
{
    register char *p = string;

    while (1) {
    if (p[0] == 0) {
        return p - string;
    }
    if (p[1] == 0) {
        return p + 1 - string;
    }
    if (p[2] == 0) {
        return p + 2 - string;
    }
    if (p[3] == 0) {
        return p + 3 - string;
    }
    p += 4;
    }
}

The link is this. I don't understand why the steps are 4. The function jumps 4 bytes by 4 bytes over the string. Why it does this way? It could be implemented like this:

int
strlen(string)
    char *string;       /* String whose length is wanted. */
{
    register char *p = string;

    while (1) {
    if (*p == 0)
        return p - string;
    p += 1;
    }
}

Is there any performance related reason behind it?