I installed mysql-5.1 into my server and mysql-4.1 has been installed already. Then, I realized that I can connect from mysql-5.1 client to mysql-4.1 server like this.
[hoge@localhost:/usr/local/mysql5/bin]% ./mysql -u root -pXXXXXX -P 3306 -S /tmp/mysql4.sock
I can use select query to mysql-4.1 server. Is there any restriction in this case? If there is a official document, please tell me.....
mysql5 has a perfect backward compatibility?
The one thing you have to worry about is what you're using to access your MySQL 4.1 database. When using PHP 5.3.0 with MySQL on the Microsoft Windows platform, I've hit a connection issue relating to the difference in password hashing between 4.1 and 5.1. See the official docs from MySQL on password hashing.
It's not fully backwards compatible. Read this document for the changes, in particular the ones marked Incompatible change.
Simple queries may work fine, but is much better to use the correct client for the server version (this goes double if you are accessing the server programmatically)