Several visible behaviors have changed between MySQL 4.0 and MySQL 4.1 to fix some critical bugs and make MySQL more compatible with standard SQL. These changes may affect your applications.

Some of the 4.1 behaviors can be tested in 4.0 before performing a full upgrade to 4.1. This gives you the 4.1 behavior for the most critical changes. That way you can test the most new behaviors in 4.0 to make sure that your applications work with them. This helps you have a smoother transition when you perform a full upgrade to 4.1 later.

3 Way to run 4.0.12 or above in this 4.1 "simulation" mode:

1) From the command line:
We have added to later MySQL 4.0 releases (from 4.0.12 on) a --new startup option for mysqld. See Section 5.2.1, “Command Options”.

2) For a particular client:
 You can also enable these behaviors for a given client connection with the SET @@new=1 command, or turn them off if they are on with SET @@new=0.

3) In the my.ini file (the one I'm running):
[mysqld-4.0]
new

Putting the --new option in the [mysqld-4.0] option group ensures that you do not accidentally later run the 4.1 version with the --new option.