$ ./a.out inputfile >outputfile

#include <stdio.h>
#include <stdlib.h>

#define NPLANE 4

int main(int argc, char *argv[])
{
  static unsigned long count[NPLANE][256] = {0};
  int c, plane, i;
  FILE *fp = fopen(argv[1], "rb"); /* エラーチェックしてない */
  for (plane = 0; (c = fgetc(fp)) != EOF; plane = (plane + 1) % NPLANE)
    count[plane][c]++;
  for (i = 0; i < 256; i++) {
    printf("%d,%lu", i, count[0][i]);
    for (plane = 1; plane < NPLANE; plane++)
      printf(",%lu", count[plane][i]);
    printf("\n");
  }
  return 0;
}