It was a common way back then of saying how long it had been since the United States had been born.
A score is equal to 20 years, so 4 score and 7 years is 87 years.
This phrase comes from the Gettysburg Address, a speech given by US President Abraham Lincoln on November 19, 1863. He gave the speech at the dedication of the Soldier's National Cemetery, near the site of the Civil War's Battle of Gettysburg which took place several months before.
The speech begins, "Four score and seven years ago...", referring to the year 1776 (87 years before the speech was given) when the Declaration of Independence was signed.
