The documentation page: http://ez.no/doc/ez_publish/technical_manual/4_0/installation/the_setup_wizard
Says the following:
"
Note that if the MySQLi extension is enabled in PHP, the "MySQL Improved" option will be available on the list. If you are going to use a MySQL database, it is recommended to select "MySQL Improved" instead of "MySQL". "
As Piotrek asks for, it would be great with some details of the difference between using mysql vs. mysqli in the context of eZp 4.
If you know, please share your information here :-)