Why does Apple not sell the series 4 watch.

The feature differences between the Series 4 and Series 5 were not all that big. From a marketing stand point it made more sense to make the Series 3 the less expensive model, and the Series 5 the top of the line model.

Also the physical dimensions and manufacture for the Series 4 and Series 5 were very very similar so it was easy to switch Series 4 manufacturing lines over to Series 5 and crank out millions of Series 5 Apple Watches in a short amount of time so they would be ready when it was announced.

PS. I own a Series 4 and it is a perfectly fine Apple Watch. I do not have any issue with it, and I'm perfectly happy Not upgrading to a Series 5.

No flaw, it was simply discontinued when the 5 was released.