ResolutionThe short answer is 'no'. For web applications Parallels Plesk Panel uses PHP that is loaded as apache DSO module. It is very difficult to have both php4 and php5 modules in apache simultaneously, because of common namespace.
In general possible solutions for having both PHP4 and PHP5 are for example:
- run two independent apache copies one for each PHP module,
- run one of PHPs as module and other one as CGI/suPHP
- compile php modules with manual special customizations in module sources to prevent their intersection.
None of these options was tested with Parallels Plesk Panel and none is officially supported by current versions. Plesk supposes that system has standard apache package that loads standard PHP module (which can be single only). Note that neither PHP nor apache web server are parts of Parallels Plesk Panel software, it uses standard services which are installed on the server (in most cases these are standard packages from operating system vendor).