apple watch series 4

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

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.

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.