Version 4.0 introduced the Physics component and changed how TranslateTimeline works. While powerful, that required rewriting hundreds of animation controllers for existing projects.
Many web-based games still rely on the 3.8.99 export format because it offers a perfect balance between file size and visual fidelity. Spine 3.8.99
Skin and attachment handling
Directly opening a 3.8.99 project in Spine 4.2+ requires upgrading the exported data. The editor will convert it, but the process is irreversible. Runtimes across the project (animation system, loading, rendering) must all be updated in lockstep. Version 4
In the Spine ecosystem, a version like 3.8.99 is designated for Spine 3.8.99