87The term "score" refers to a unit of twenty years; four score, is 4 20s or 80 years, and 7, is 87 years. 87 years, prior to 1863 (the year the Gettysburg Address, was given.), was 1776, the year of...
A score is 20, so four score would be 80, plus 7. Four score and seven years ago would be 87 years ago that Lincoln is referring to in his Gettysburg Address