After installing MySQL 4.0 I got the following error:
Fatal error: Can't change to run as user 'mysql'. Please check that the user exists!
As described here, it is solved by editing /etc/init.d/mysql and inserting a sudo into the start case:
sudo -u mysql $bindir/mysqld_safe --datadir=$datadir --pid-file=$pid_file >/dev/null 2>&1 &