Fascinating discussion.

I just went though all of my Amex cards - I have 23 with all the "additional" card members on SPG, HH & DL cards.... not one of them has the same 4 digits... although often they are just a digit or two away.

Perhaps worth calling to ask for some new cards??

Still... then you have to ask yourself... do you want to gain on one card and lose on another?

Buy on Credit, return on Debit would be the way to go but none of the last 4 digits are close to the Amex figures.

To irritate things further... I have a UK and US Amex with the same last 4 digits... so a swap could be done... but then you're dealing with exchange rate fluctuations and commission fees!

It just isn't worth it.