California has no limit on how far they can go back. California has 4 years from the date you filed your return to issue an assessment. The only exceptions are

California has no limit on how far they can go back. California has 4 years from the date you filed your return to issue an assessment. The only exceptions are for substantially underreported liability, which is considered fraud, or where a tax return was not filed. In such cases the statute of limitations is indefinite. The part where you say "mostly W2" indicates that you had a number of non-W2 sources of income, which is probably where the underreporting arose. (A lot of people fail to properly report 1099 income. A lot of crypto traders fail to properly report crypto sales.)