apple watch series 4

It could just be a manufacturing resource decision. They already have to tools and dies for making the smaller Series 3 cases, but since the Series 4 and Series 5 Aluminum and Stainless Steel cases are the same size, they would have to either build out more manufacturing capacity to satisfy both Series 4 and Series 5 sales and that may limit initial supply. By keeping the Series 3, they can use that existing manufacturing capacity and switching the Series 4 manufacturing equipment over to Series 5 means they already have the capacity to build the Series 5.

Also there are not a lot of differences between the Series 4 and 5, unless of course you are one of the people that really wanted the "Always On" capabilities, or the large storage capacity.

Not directly from the Apple site.

You can try 3rd party retailers still have Apple Watch Series 4 inventory

And online retailers such as Amazon.com

Nobody at Apple will say a word. I'm assuming the 4 & 5 are so similar that if they kept the 4 available the 5 wouldn't sell as many as they want.