Hi Lindsey,
ZPPO04 seems a custom transaction code.(I cannot find this transaction in my system). Hence you can add your code at the point where you want to send notification in logic.
There are multiple options of achieving the results. Quickly i can think of two.
1. Sending mail from ABAP using CL_BCS and CL_BCS_Document Classes.
2. Calling workflow that will have email/notification step for pushing the mails.
Regards,