It would be my pleasure to provide you insights about 401k form in QuickBooks Online.
A 401k deduction reduces wages for federal income tax and most state income tax withholding. For more information, check out this article: Types of Retirement Plans.
With this, I suggest contacting your state for more guidance in filing the forms. Use this article to get their phone number and directly go to the Withholdings section: California Payroll Tax Compliance.
Feel free to leave a comment below if you have other concerns. I'm always here to help.