A judicial proceeding in which the accuracy of the public-offering statement or validity of any contract of purchase is in issue or in which a rescission of the contract or damages is sought shall be commenced within four years after the date of the contract of purchase, notwithstanding that the purchaser’s terms of payments may extend beyond the period of limitation except that, with respect to the enforcement of provisions in the contract of purchase which require the continued furnishing of services and the reciprocal payments to be made by the purchaser, the period for bringing a judicial proceeding will continue for a period of four years for each breach, but the parties may agree in writing to reduce the period of limitation to not less than two years.