AlterVista supports both PHP4 and PHP5.
By default PHP4 is enabled and PHP5 is disabled. You can enable PHP5 by accessing your panel in the section AlterSite->File management and clicking on the gear in the left-top of the page. By doing this, you enable PHP5 on the directory currently displayed in the file manager.
So you can enable PHP5 on directories containing PHP5 scripts, without affecting directories containing PHP4 scripts.