Why not simply using:
- $4 has anonymized an account. New username: $5.
It makes clearer that the anonymized account is NOT the account owned and used by $4 perforing the change, but that the admin $4 made the change in the other account now named $5 (whose previous name is voluntarily hidden, but may be found by a few admins using restricted privacy inspection tools). I suppose that this message is the one that all other users will see in the public history log of the new user account (whose name ion $5 may look like "User:Anonymous-71ef38a9" as it was probably randomly generated with some name generator, and checking it did not cause conflict with another active account, or possibly selected manually by the admin doing the check himself).