"Never happened to me. Your all liars. Continental makes a fine tire!" (a typical response on these threads).
I have blown out a sidewall on these (after countless flats) and the tire is likely trash now. I was riding on gravel at the time, and assume it was a sharp rock but I never found the object.
I think rather than flat out denying the issue exists, people running GP 4000's should at least be aware that the sidewalls on these tires are not designed for anything more challenging than perfect pavement that's swept regularly (which rules out every single road in CA). Riding these on gravel is flat-out asking for trouble.
But other than that, these are some great tires, worth every penny. Just be aware of their fragility over many common surfaces, and stick to good pavement whenever possible.