they had obviously priced the 4 pack at the same price of 1 can!
I am by no means a plaster saint, but I would worry about that, if it happened to me, until I had contacted the retailer. Since starting to use Ocado for grocery deliveries, we have had a couple of over deliveries, and one case where they refunded more than the cost of a missing item, and I emailed them each time. The latter incident elicited a cheery e-mail and a £5 voucher 'for being honest'.
In the 1980s, I lived in a seedy part of Bristol, and one day the shopkeeper gave me change for 50p when I had given him a pound. He would not be convinced. I went away seething (it was a short time for me financially) but a few weeks later, I accidentally dropped a 20p coin and it rolled under the counter (which was on stubby legs, there was a gap of about three or four inches). The same (oldish guy) said he couldn't bend over to get it so I'd have to. I got down and peered under the counter. I saw my 20p and, covered in varying amounts of dust and fluff, several pound coins! I managed to retrieve the lot with one motion of my hand, and brandished the 20p happily.
In the 1990s I was in my local corner shop in another part of Bristol and the guy gave me change for £20 when I had only given him £10. I told him at once and he got a little bit shirty, accused me of telling him 'how to do his job' and insisted that he had given me the right amount of change. I think there had been drink taken. Over the next few years we got quite pally and I told him about that incident, and he laughed and said he 'must have been on the beer'.
Happy days!