Any action brought to enforce the provisions of this article shall be barred unless commenced within four years after the cause of action arose, or if the cause of action is based upon a conspiracy in violation of this article, within four years after the plaintiff discovered, or by the exercise of reasonable diligence should have discovered the facts relied upon for proof of the conspiracy. For the purpose of this section, a cause of action for a continuing violation is deemed to arise at any time during the period of such violation.