Apple Watch 4

The Apple Watch Series 4 is just like last years model car. It still exists. It can still be used. Its warranty is still in force. Apple Care purchased for it is still valid. It can still be serviced. It can still be upgraded to watchOS 6 next month. The electricity used to charge it is still compatible, etc...

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They do not sell the original 1st generation, nor the Series 1 and Series 2 Apple Watches.

The 1st Gen stopped getting watchOS upgrades at version 4, but the Series 1,2,3 and 4 line all were able to upgrade to watchOS 4

Non-Apple websites say the Series 1,2,3 and 4 models will also be able to upgrade to watchOS 6.

Just because Apple stops manufacturing a given product, does not mean it no longer exists in customer hands, and that they have stopped supporting the product.

Eventually the older models will stop getting operating system upgrades, which is generally because of hardware limitations (it is too slow, not enough operating RAM, not enough storage to hold the operating system and apps, there is some chip restriction that the new OS needs, etc...).

The context is that the 4 seems to have been wiped from Apples mind. You can buy the 3 and order the 5 soon, but the 4 is now missing.

Yet Apple is still selling the 3. Bit strange to do that.

Not really, they have done this before and not jus with the Watch. The Series 5 replaces the Series 4 and the Series 3 becomes the budget model.