California Code, Revenue and Taxation Code - RTC § 18526

(a) After the expiration of four years from the last date prescribed by law for filing the return for the taxable year (determined without regard to any extension of time granted to either spouse).

(b) After there has been mailed to either spouse, with respect to the taxable year, a notice of deficiency under Section 19033, if the spouse, as to that notice, files a protest under Section 19041 or appeal under Section 19045.

(c) After either spouse has commenced a suit in any court for the recovery of any part of the tax for that taxable year.

(d) After either spouse has entered into a closing agreement under Section 19441 with respect to the taxable year.