The last four digits of your physical credit card number are stored by Apple Pay for the purposes of account identification.

There are varying reports of people seeing unfamiliar (i.e. device) numbers and their card number on receipts. I've no knowledge about exactly why there's no consistency, but my suspicion is that Apple Pay sends these four digits as metadata during the contactless transaction. Older contactless terminals would ignore it and naively print four digits of the account number, whereas newer terminals might recognise that metadata and print that on the receipt instead.

It's also possible that the physical card's last four digits are being sent back to the terminal by the network.