Can I have an online calendar alongside my email account?

All users with email accounts have access to Radicale, a very simple tool that allows you to create calendars that you can then interact with using the CalDAV protocol. There are many CalDAV clients available - for example, if you use an Android phone, the DAVx5 app will bring your 4d2.org calendar into your phone as an additional calendar source alongside your Google Calendar account. You can then use any calendar app you like to add, edit or remove calendar entries.

Your login credentials for Radicale are the same as your email credentials.

Radicale has some limitations and does not support some more complex CalDAV scenarios.