Final answer:
An employer must retain an employee's W-4 form for a minimum of four years after the tax due date for the year the form was last used in accordance with IRS guidelines.
Step-by-step explanation:
The question, "How long must an EE's (employee's) W-4 be retained by an ER (employer)?" pertains to the recordkeeping requirements for tax forms in the United States. According to the Internal Revenue Service (IRS) guidelines, employers must retain the W-4 forms of their employees for at least four years after the tax due date for the year in which the employee last provided a W-4 form. Therefore, the correct answer is C. 4 years.