4. No insurer shall fail to renew a policy unless it shall mail or deliver to the named insured, at the address shown in the policy, at least 60 days' advance notice of its intention not to renew, except that the commissioner may extend the advance notice period up to an additional 30 days by regulation. This section shall not apply:
(a) If the insurer has manifested its willingness to renew; nor
(b) In case of nonpayment of premium;
provided that, notwithstanding the failure of an insurer to comply with this section, the policy shall terminate on the effective date of any other insurance policy with respect to any automobile designated in both policies.
Renewal of a policy shall not constitute a waiver or estoppel with respect to grounds for cancellation which existed before the effective date of such renewal.