If you signed the contract 4 years ago and they never sued you, you can ignore their phone calls altogether.  The statute of limitations (period in which a person or company can bring a legal claim) for written contracts is 4 years in CA.  See Cal. Civ. Proc. Code § 312 et seq.  After 4 years, its over.  Their right to collect on the contract is dead.  So unless these people sued you and have a judgment out there, you can ignore their calls.