On Washington's Walk: Fort Four historical marker: “Fort Number Four of the Exterior Defenses of Fort Washington and King's Bridge Constructed by the American Army under General George Washington, 1776.“ The plaque was installed by the Daughters of the American Revolution and bears the date 1916. The Old Fort Four Park page New York City Department of Parks Website[1] says “The site is marked by a flagstaff and a bronze tablet erected by the Daughters of the Revolution in 1914.” There's some confusion about the year, but this is indeed the bronze tablet. However, no flagstaff is to be found in the area.

References:

1. Washington's Walk: Old Fort Four Park on the New York City Department of Parks Website.