Hosting provider 1&1 is “deactivating” MySQL 4 databases starting on May 1st, 2013. All WordPress sites hosted with 1&1 that have not migrated their databases from the old database to the new MySQL 5 databases may have their sites go offline, however 1&1 doesn’t explicitly say the sites will go down. If you host a database driven site on 1and1.com, log in, go to MySQL Administration and check what version of MySQL your site is using.

To quote directly from the 1&1 announcement:

The information below applies only to Linux Hosting packages with MySQL 4 databases. MySQL 4 is being phased out and therefore any data held within a MySQL 4 database must be migrated into a new MySQL 5 database.

Why is the migration not done automatically?
The migration can not be carried out automatically because of some features of MySQL 4 are not supported in MySQL 5 and in order to minimize the amount of transmission and handling of your private data. Because of this we are allowing customers to carry out the migration on their own, at their leisure.

How do I know if my databases are affected?
Any database created in the 1&1 Control Panel that says MySQL 4 as the version, will need to be migrated to a newly created MySQL 5 database. You can check what version of MySQL your databases are by logging into your 1&1 Control Panel and clicking on MySQL Administration from the Domains & Web Space section.

Full instructions on upgrading your MySQL version on your 1&1 hosting account can be found here: http://faq.1and1.com/web_space__access/mysql/20.html