@WorzelWould have been better promoted as buy 3 & we give you the 4th one free. At the moment it looks like buy 4 and get 1 free = 5 shows (Just my opinion) :-)
the server has to detect a 4th purchase, and then you get Credit back for that 4th card.
So it is Pay for 4, and get back the credits for the 4th card.
you are getting 4 cards, for the price of 3, the 4th card is free.
But technically, if you then Spend the refunded credits on a 5th card, then you got 5 cards for the price of 4,
and if you buy a 6th card, then you got 6 cards for the price of 5...
if you buy a 7th card, then you get 7 cards for the price of 6...
and If you buy an 8th card, then you get refunded for the 8th card, and Now you have 8 cards for the price of 6.
and that works out to Pay for 3 and get the 4th one free.