Hello and welcome to Apple Discussions.
While Canon may have stopped supporting the S400, the Gutenprint driver suite still lists this model as a supported model.
Gutenprint drivers can be added on install of SL by doing a custom install.
Or, if you have the OS installed already, you can add them via the SL DVD, selecting the Optional Installs package.
Or, you can download the latest v5.25 from Sourceforge.
http://gimp-print.sourceforge.net/MacOSX.php
When you physically connect the printer's USB cable to the Mac, I believe the Gutenprint driver will be automatically selected. If not, you can open the Print Using menu, click 'Select Printer Software' and type S400 to shorten the list. The Gutenprint driver will show as "Canon S400 - Gutenprint v5.2.5". Select this and click OK and then Add to complete the print queue creation.
Pahu