v4 upgrade doc, changelog for 4.0

docs/source/administrator/upgrading/upgrade-3-to-4.md

@@ -2,4 +2,84 @@
 
 # Major upgrade: 3.\* to 4.\*
 
-This is currently just a placeholder file for until we have written upgrade docs.
+zero-to-jupyterhub 4.0 is a major upgrade that may require some changes to your configuration,
+depending on what features you may use.
+This mostly comes in the form of some upgraded packages, described below.
+
+See the [changelog](changelog) for details of upgraded packages.
+
+## JupyterHub 5
+
+zero-to-jupyterhub 4.0 upgrades the JupyterHub version from 4.1.6 to 5.2.1.
+
+:::{seealso}
+For more detailed changes in JupyterHub, see JupyterHub's own documentation on upgrading to version 5:
+
+Especially if you use features like per-user subdomains or custom page templates.
+
+- [jupyterhub changelog](https://jupyterhub.readthedocs.io/en/5.2.1/reference/changelog.html)
+- [Upgrading to JupyterHub 5](https://jupyterhub.readthedocs.io/en/5.2.1/howto/upgrading-v5.html)
+
+:::
+
+### Allowing users
+
+JupyterHub 5 promotes the `allow_all` and `allow_existing_users` configuration used on OAuthenticator to all other Authenticators.
+If you have no explicitly allowed users, the default is now to allow no users (this was already the default if you were using OAuth).
+If you are using an Authenticator where all users who can successfully authenticate should have access, set:
+
+```yaml
+hub:
+  config:
+    Authenticator:
+      allow_all: true
+```
+
+to explicitly opt in to this behavior that was previously the default for some authenticators.
+
+## KubeSpawner 7
+
+zero-to-jupyterhub 4.0 upgrades the KubeSpawner version from 6 to 7.
+The main relevant change here is the "slug scheme" used to compute names of resources such as pods and PVC (user storage) is changed.
+
+The new scheme is called "safe" and is the default, whereas the old scheme is called "escape".
+If you do not have any custom templated fields, it is _unlikely_ that anything should change for you and everything should work.
+If, however, you specify custom templated fields such as volume mounts or environment variables,
+especially those that have the `{username}` or `{servername}` fields,
+those values are likely to change under the new scheme for some usernames.
+In particular, there are new fields that should make things easier:
+
+- `{user_server}` combines the username and server name, and is equivalent to `{username}{servername}` in the old escape scheme
+- `{pod_name}`, `{pvc_name}`
+
+You can opt in to the kubespawner 6 behavior with:
+
+```yaml
+hub:
+  config:
+    KubeSpawner:
+      slug_scheme: escape
+```
+
+which _should_ result in no changes for you from previous behavior.
+
+:::{seealso}
+
+- [KubeSpawner changelog](https://jupyterhub-kubespawner.readthedocs.io/en/latest/changelog.html)
+- [KubeSpawner docs on templated fields](https://jupyterhub-kubespawner.readthedocs.io/en/latest/templates.html#fields)
+
+:::
+
+## OAuthenticator 17
+
+OAuthenticator is upgraded from 16.3.1 to 17.1.
+The main changes are related to using group information from OAuth providers.
+If you used or would like to use groups for authentication,
+check out the [OAuthenticator changelog](https://oauthenticator.readthedocs.io/en/stable/reference/changelog.html#:~:text=17.1)
+
+## Other package upgrades
+
+- Python is upgraded from 3.11 to 3.12 in the Hub image
+- LDAPAuthenticator is upgraded from 1 to 2 ([changelog](https://github.com/jupyterhub/ldapauthenticator/blob/2.0.0/CHANGELOG.md#200---2024-10-18))
+- FirstUseAuthenticator is upgraded from 1.0 to 1.1 ([changelog](https://github.com/jupyterhub/firstuseauthenticator/blob/1.1.0/CHANGELOG.md))
+- idle culler is upgraded from 1.3.1 to 1.4.0 ([changelog](https://github.com/jupyterhub/jupyterhub-idle-culler/blob/1.4.0/CHANGELOG.md))