In Spine 4.x, the ease in and ease out buttons were removed from the graph toolbar, and instead, the Curves view was introduced to manage curve presets. The Curves view allows you to define and save your own custom curves, including ease in and ease out types. To use the Curves view, you can create a curve by adjusting the Bezier handles to your desired shape and then save it as a preset. This way, you can apply these custom curves to your animations as needed.

To bring back ease in and ease out functionality, you can manually create these curves in the Curves view and save them as presets. Once saved, you can apply these presets to your animations. Although there isn't a detailed guide specifically for the Curves view in the official documentation, you can experiment with creating and saving curves to get the desired easing effects.

Dora Sorry for the delay in updating the Spine User Guide! A general overview can be found in the blog post when Spine 4.1 was released:
https://esotericsoftware.com/blog/Spine-4-1-released#Curves-view

The Presets section is collapsed and not displayed by default, so open the options menu from the button in the top right and check the Presets box to display it:

When we add new features, it can take some time for the corresponding explanations to be added to the User Guide, but the tooltips that appear when you hover your mouse over each button or other UI element are usually added at the same time as the feature is added, so if there is a feature you don't know how to use, please refer to the tooltip: