CA Rev & Tax Code Section 5096.5

Any taxes paid which were not erroneously or illegally collected under the law as it existed at the time of collection, but for which an exemption is provided by a retroactive constitutional amendment, shall be refunded after compliance with the provisions of this article, except that the claim for refund may be filed at any time within four years after the date such amendment became effective, or the date that this section became effective, whichever is later.