You'll need to buy them now from Royal Mail, as the other option would involve cancelling all 4 orders, refunding, relisting for the buyer to buy again, too much hassle.
You cannot now add any new tracking, eBay suggest sending the buyer a message with those tracking details. If a case is opened later for item not received, you will be able to add those tracking details.
A refund is sent to whoever paid for the SD label:
- If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days
@golden_frog