How to repeat an API call every 4 hours until data is available

I have a scenario triggered by a webhook. My goal is to send a guest an email after their reservation is confirmed. The challenge is that Airbnb doesn't automatically provide the guest's email. When a reservation is confirmed, a webhook is sent without the email address, requiring me to request it separately.

Consequently, I have several routers set up following the webhook, depending on whether we have their email. One of these routes involves an API call to check if they've provided their email yet. I want this check to repeat every 4 hours until an email address is available, after which the guest will receive an email.

What would be the most effective approach for this?