?4 is the cash fare, a single can cost ?2 or ?1.50 (depending on time of day) if you use PAYG on Oyster. The exorbitant cash price is a mechanism to get people to switch to Oyster PAYG instead of buying paper tickets. Same with buses: 90p on Oyster, ?2 cash.

Jon14: re receipts for PAYG fares - I think there's a way of printing out a mini statement or something at Oyster ticket machines. Never done it myself but it's come up in conversation at work because of people needing to reclaim fares.