You can find out the version of different programs that Umbrella Host run by logging into your eXtend Control Panel. Select the Web Diagnostics section and then select 'Paths and Versions'.
You can use different versions of PHP by changing the extension on the script in question. To use PHP4 with a script, leave its extension as .php. If you want to use PHP5 with a script, change the extension to .php5.
If you wish to default all .php scripts in your hosting space to use PHP5, but do not want to change the extension, place this line into a .htaccess file in your FTP space
SetEnv DEFAULT_PHP_VERSION 5
Both PHP 4 and 5 are available on Umbrella Host's servers