Form SS-4 is used to apply for an employer identification number—or EIN—which the IRS uses to keep track of businesses the same way it uses social security numbers to keep track of individuals. You need an EIN if you pay wages to any employee.
Many individuals who don't think of themselves as an employer need an EIN because they employ household help, to care for children, perhaps, or do housework.
Even if you don't have employees, you need an EIN if you are self-employed and have a Keogh retirement plan.
If you apply for your EIN by mail, you should do so at least a month before you plan to file your return. It will take that long for the IRS to assign a number.
A faster way to file for an EIN is under the Fax-TIN program, you will receive you EIN within 4 business days. Just complete and fax Form SS-4 to the IRS using the phone number found on the IRS website www.irs.gov/pub/irs-pdf/iss4.pdf, page two of six of the Instructions for Form SS-4.
The two fastest ways to receive an EIN is to use the Online program at www.irs.gov/businesses and click on Employer ID Numbers. Using this method, will get you an SS-4 number immediately. (Certain restrictions exist for unusual situations.) Or call the IRS at 1-800-829-4933. When you call, you should complete Form SS-4 prior to calling and read it to them over the phone. The person calling must be authorized to sign the form. If requested by the IRS representative, mail or fax a signed Form SS-4 within 24 hours to the IRS address provided by the IRS representative. Again, you will be given an EIN over the phone, so you can use it immediately, and you should write the number in the upper right hand corner of the Form for your records.
Don't send it along with your tax return. IRS instructions for the form have the right address for your area, and in most cases the address is different from the address where you mail your tax return.