An independent agency will review an estimated 4,000 drug cases in New York after officials from a medical examiner's office uncovered errors at a police crime lab. The possibility does exist, defense lawyers contended, that innocent people have been jailed in drug cases based on faulty evidence from the lab. Already, 16 defense motions seeking judicial reviews or reopening of drug cases have been filed, according to the DA. More are likely. "The credibility of the entire lab has been undermined," said Marc Gann, president of the Nassau County Bar Association.