As you are aware this the way Amex's last four digits work, but for anyone else - they're much more likely to be identical than with the other card brands. The way they work is that they only include the issue number (as the 12th digit) and the checksum (as the 15th). 13 and 14 are always zeroes. As you will always start from an issue of 1 there are very few possible Amex last 4 digits...

I agree with you that sooner or later Amex will need to consider renumbering.