I've had 4 punctures in the last 2 weeks while commuting on my not-very-old GP4000s - including 2 punctures on the way in yesterday which meant I had to wheel my bike 2 miles to work having used up my last spare tube - and have come to the conclusion that they are rubbish...
I've just bought some trusty 4 Seasons instead, and am expecting much longer/better life from them
So I guess this post is just a warning to avoid 4000s