I think that there could be confusion about whether Playground is actually Workspace... so why not name the item in world menu as "Workspace / Playground", so it's obvious it is both?
Also don't forget that there is keyboard shortcut which now opens Playground instead of Workspace; so some amount of confusion will probably happen anyway. Or maybe have a way to inform the user about the new changes... so maybe a new world menu entry "Pharo 4.0 news/changes/whatever"? Or just put some release notes on Pharo.org.
If someone opens new MAJOR release then changes and BC breaks are to be expected, otherwise what would be the point of a major release.