This doesn't necessarily confirm that it's 4, though. For example, when I write a new version of a package on some package management system, the code may be updated by 1 major version but it stays the exact same version until I enter the new version into the manifest. Perhaps that's the same here; the training and architecture are improved, but the version number hasn't been ticked up (and perhaps intentionally; they haven't announced this as a new version openly, and calling it GPT-2 doesn't explain anything either).