PHP 5.1 and MySQL 4.1

[Matt]

You can convert to 4.1 if you like, but the conversion is not backwards compatible so if you think you might want to go back to 4.0 (I don't see why there would be a need after a several successful upgrades) you should have your backups ready.

Most of your users will have authentication issues with their applications because the login style to mysql has changed.
To deal with that:

1. You can enable old style passwords or
2. Use the new style passwords and go into the config.php files and update them for the new format.

My vote is number 2, for numerous reasons.