Cal. Rev. & Tax. Code § 12978

No credit or refund shall be allowed or approved after four years after April 1st of the year following the year for which the overpayment was made, or with respect to a deficiency assessment made under Article 3 (commencing with Section 12421) of Chapter 4 after six months from the date the deficiency assessment becomes final, or after six months from the date of the overpayment, whichever period expires the later, unless a claim therefor is filed with the commissioner or the board within that period.