Cal. Rev. & Tax. Code § 19065

If any taxpayer agrees with the United States Commissioner of Internal Revenue for an extension or renewals thereof of the period for proposing and assessing deficiencies in federal income taxes for any year, the period for mailing a notice of a proposed deficiency shall be four years after the return was filed or six months after the date of the expiration of the agreed period for assessing deficiencies in the federal income tax, whichever period expires the later.