Recurring Webinar Series setup help

You could set up a recurring webinar series in Zoom and manually update every other occurrence to be a second daily slot. That way they will only have to register once for the 4 day series. If you want to send recurring calendar invites to your registrants that will include all the sessions on the calendar there is a Zoom app, Salepager, that lets you send recurring calendar invites with the Zoom link upon registration.