Apple Watch series 4
Why was the Apple Watch series 4 discontinued?
The manufacturing line was switched to making Series 5 which is structurally very similar to the Series 4.
And the Series 5 manufacturing line (which used to be the Series 4 line) was switch to making the Series 6.
Also Apple does not like too many similar items in the same product line, especially if the differences between specific models is not all that great.
The Series 3 is a different case size so its manufacturing line was not as easily converted, and the feature set in the Series 3 is sufficiently different that it provides a clear selection different and a justification for the prices difference.