I just had a client sued by a debt collector for a debt that was more than 4 years old. The complaint (the lawsuit in lay person terms) stated that the debt was delinquent in February, 2004. The lawsuit was filed in August, 2008. I filed a demurrer which is basically a motion to dismiss. The court granted my motion.
Remember, California has a 4 year statute of limitations from the date of last payment or last charge. That means the lawsuit must be filed within 4 years of that date. The debt collector cannot think about filing it within 4 years. The debt collector cannot attempt to file it within 4 years. The debt collector must actually file the lawsuit within 4 years. Failure to do so means you can beat the lawsuit.
Of course, note that even if you beat the lawsuit, the debt can stay on your credit report for 7 years from the date it became delinquent. But, small wins are better than no wins!