ResolutionThe latest Plesk versions starting from 8.1 support both PHP 4 and PHP 5. Both versions are shipped with Plesk and can be installed as Plesk components via Add/Remove programs, Plesk, Change. Once both versions of PHP are installed, the required version for each individual domain can be selected via Physical hosting configuration page Plesk control panel, Domains, domain.tld, Setup.
So the domain itself can be configured to use one PHP version. It cannot use two PHP versions simultaneously.
However different PHP version can be configured on virtual folder hosted by this domain.
To make so it is necessary to tick Create Applications checkbox on Web Directory creation stage ( Plesk, Domains, domain.tld, Web Directories, Add New Virtual Directory page ).
Once virtual directory is created it is possible to enter there and go to PHP version area (Plesk, Domains, domain.tld, Web Directories, _your_virtual_directory_, PHP version). PHP version for current virtual directory can be configured in this area and can be different than PHP version of parent domain.