An employer identification number (EIN) is the business equivalent of a Social Security number. A unique identifier is assigned to the business for use in communicating and complying with the Internal Revenue Service. Just like a Social Security number, it is a 9 digit number with the first two digits as the prefix.
The SS-4 is an application form to the IRS requesting an identification number for a non human entity (businesses, estates,trusts, partnerships, etc.).
When an individual passes away, his/her will or trust identifies a representative to administer his/her estate. This representative is referred to as the executor (male) or executrix (female) and is generally approved or assigned by the local circuit court. The Internal Revenue Service tasks this representative to file a final personal return and information returns until the estate is completely transferred to the heirs.