The goal of the antibiotics is to prevent the infection from killing you while buying time for your immune system to kick in and handle things.
Secondly, why would a second course be more harmful if the duration of the two courses combined is less total time than the initial prescribed amount?
If you feel better after 4 days of antibiotics, stop, get sick again and take 4 more days of antibiotics, you're still only dosing for 8 days. That's far better than a default of 2 weeks (14 days), and only barely worse than a 7 day course.
Now factor in the number of people who would have been just fine after 4 days, with no re-occurrence, and I'm really struggling to see why your advice is any better than: Treat when sick, stop when well. Even if you get sick again: treat when sick, stop when well.