When your account was set up, it sounds like you were set up for PHP4 for some reason. As far as I know, there is no way for us users to simply switch PHP versions on the fly pr in the control panel.
What you CAN do (and probably what you need to do) is create a new subdomain (like, wiki.feedle.net as a personal example) in the panel, and make sure that when you configure that subdomain you select PHP5.
Assuming you have subdomains available, you can log on to the panel, select the "domain" tab, and click "Manage Domains". At the bottom of that page, there's a link to "Add new domain/sub-domain". Create a new subdomain called "wiki.domain.com", and make sure you select PHP5. You may also want to give it a unique directory.
Give it a shot..