You can easily install PHP 4 from your PlusMail Site Control panel. Login to PlusMail by going to http://your-domain.com/cgi-bin/plusmail. Click on Free Scripts on the left. You'll see a script near the bottom called PHP4. Put a check mark next to that and press Install Scripts.

Then you can go back to http://your-domain.com/webmail and it will show up.

PLEASE NOTE: that when you leave an email in the webmail Inbox it remains on the mail server. This means that after awhile your mailbox on the mail server will fill up. So you must either delete messages from the webmail Inbox or move them into another folder. By creating other folders and moving messages you want ot save into those folders, you will also be removing the messages from the mail server and freeing up space in your email account.

IMPORTANT: The first time you use WebMail, you should click on Options at the top of the screen and configure it as you like. The most important options you need to set are your reply-to address (this is the e-mail address people will see when you send them a message). Full Name will set the Display Name on your e-mails. If you don't set these up all your e-mails will appear to come from your-domain@your-domain.com. You may want something more specific like sales@your-domain.com. To set your e-mail address in WebMail click on Personal Information under Options. You can also set your time zone here so mail will appear as received in local time.

Also, another couple of important settings you should make in WebMail are under Display Preferences.

You should select Yes for "Show HTML Version by Default", and Yes for "Display Attached Images with Message".