It's simple. Don't drive the car any faster then you plan to go for your next accident. And only drive your car in places where you'd like to have that accident. After all, it's only your safety that is at risk, here.
I'm sure the sales rep jumped towards the DOT date code; they're all trained on that and they think drivers have no idea. Am quite confident the 4 year number is accurate.
Thanks for all the replies. The DOT code says 1612. Very disappointing that these only lasted 3.5 years.
No unusual usage or abuse. I did notice they lost air pressure rather quickly, it must have been coming through the sidewalls. Guess I'll have to bite the bullet and replace them all.