Does Form W-4 let an employer ignore withholding?

No. Form W-4 gives the employer the employee inputs used to compute federal income tax withholding. A valid Form W-4 does not give the employer permission to ignore the withholding calculation.

Original example:

Sierra submits a valid revised Form W-4 with an extra 80 per paycheck. The employer cannot simply say the payroll system is easier without the change. The revised form must be put into effect under the timing rules unless an exception applies, such as an invalid form or an IRS lock-in letter.

For exam purposes, identify the status of the form first: valid, invalid, exempt, lock-in controlled, or missing.