All the deprecated code introduced on 3.x is removed on 4.0.
Please read 3.x upgrade guides for more information.
See also the diff code.
The google auth dependency is optional now, please add it to you composer file if you need this.
FOSUser is now on its 2.0 version. You can check its UPGRADE notes here.
SonataUser no longer has FOSUser as its parent bundle. It requires FOSUser as a dependency always.
Removed code used to define custom User login for the frontend application. Use FOSUser directly instead.
If you have implemented a custom user, you must adapt the signature of new methods to match the one in UserInterface again