If you have opened the packet then they definately will not take it back as there is no fault with the item, its not in a re-saleable condition and they warned you before hand that you would not be able to return it
However if the memory card suddenly developed a fault and the 4gb card was out of stock, they would offer you an alternative product, you just tell them that you dont want to pay any extra for a bigger card so will settle for a 2gb one and you get the price difference on a gift card!!
Cheeky I no, but it works!