Each ewe has a 17 day cycle. This means that she will come a-tupping for one day, and then, if she is not tupped or does not hold, she will come a-tupping again 17 days later.
Therefore in a field of 4 ewes for 17 days, the tup will have nothing to do for at least 13 of those days, even if all the girls are cycling.
You can only know he has done his job with them all when there are no ewes with him for each of 17 days straight.