In order to switch between PHP 4 and PHP 5, you will need to log into your control panel at https://cp.mywinhosting.com
Next, click on the link that represents your domain name in the left panel of the window. This will lead you to the "Edit Domain" screen where you should see an item labeled, "Web Service." Click the hammer/paper icon to the right of that label.
Scroll down until you see the item, "PHP." To the right of that, you will see another hammer/paper icon. Clicking that will lead you to a screen where you can select the PHP version you want to use for your web site.