Write a function int max_of_four(int a, int b, int c, int d) which reads four arguments and returns the greatest of them.

Sure, here is a simple function in C that takes four integers as arguments and returns the maximum among them.

#include <stdio.h>

int max_of_four(int a, int b, int c, int d) {
    int max = a;

    if (b > max) {
        max = b;
    }
    if (c > max) {
        max = c;
    }
    if (d > max) {
        max = d;
    }

    return max;
}

int main() {
    int a = 1, b = 2, c = 3, d = 4;
    printf("The maximum number is %d\n", max_of_four(a, b, c, d));
    return 0;
}

In this function, we first assume that a is the maximum. Then we compare max with b, c, and d respectively. If any of these is greater than max, we update max. Finally, we return max, which will be the greatest of the four numbers.