Form SS-4 is used to apply for an employer identification number. An employer identification number (or EIN) is a nine-digit number assigned to businesses, corporations, partnerships, trusts and other entities for tax filing and reporting purposes. An individual must obtain an EIN if he or she pays wages to one or more employees.
Even if you already have an EIN, you may need to get a new one if either the organization or the ownership of your business changes. Form SS-4 handles this case, too.
You can apply for an EIN simply by filing Form SS-4. But you can also apply for an EIN by telephone using the Tele-TIN program run by the IRS. See the official instructions for the number to call. You will still need to send in a completed Form SS-4, but you can receive an EIN over the phone.
If you plan to obtain an EIN by mail, you should apply as soon as you know you will need one. The IRS recommends that you file the Form SS-4 as much as 4-5 weeks before you need it. It can take that long for an EIN to arrive by mail.
Be careful not to file more than one Form SS-4 for any one business. Sorting out the resulting confusion of having multiple EIN's can be difficult.
Don't just send Form SS-4 with your tax return (they don't go to the same places at the IRS Service Center). Please see the official instructions for an up-to-date list of mailing addresses for Form SS-4 for your area.