The following NEW packages will be installed:
  mysql-server-4.1
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
2 not fully installed or removed.
Need to get 0B/17.0MB of archives.
After unpacking 38.1MB of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 96995 files and directories currently installed.)
Unpacking mysql-server-4.1 (from .../mysql-server-4.1_4.1.15-1ubuntu5_i386.deb) ...
Aborting downgrade from (at least) 5.0 to 4.1.
dpkg: error processing /var/cache/apt/archives/mysql-server-4.1_4.1.15-1ubuntu5_i386.deb (--unpack):
 subprocess pre-installation script returned error exit status 1
Errors were encountered while processing:
 /var/cache/apt/archives/mysql-server-4.1_4.1.15-1ubuntu5_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)