The smoothness of the curves is a trade off for runtime memory usage and performance. Using more segments increases memory usage and decreases performance. It would be great if you could define the number of segments, but at runtime all curves use the same number of segments because that makes for a significant performance optimization. It's actually always worked this way, but with the 4.0 graph it's easier to see and the graph encourages you to use larger curves, where the issue is more prominent.
There are other possible solutions though. For a given key, we could allow you to specify how many extra keys you want between it and the next key. In the editor, we would then automatically calculate the ideal locations to place the extra keys to achieve the smoothest curve. You cannot modify these extra keys and they won't get in your way when working on your animation. When you export, the extra keys are exported as normal, so at runtime everything remains the same, just with more keys to make the curves smoother.
We'll consider this for 4.1, but it's a bit too early to make promises. 🙂 I've created an issue for it:
https://github.com/EsotericSoftware/spine-editor/issues/597
