Lock Four was the original community name for the lower floodplain area of present North Charleroi. The area was first settled by the Frye family in the 1770s, and between 1840 and 1844 the fourth lock of the Monongahela Navigation Company was built on the Frye farm at Frye's Shoals or Frye's Ripples after the company began constructing locks and dams along the Monongahela River in 1838. A small village grew at the lock site, with a store, post office, residences, telegraph office, and the navigation company's only boatyard, and it became an important shipping point for Fallowfield Township farmers before local rail lines arrived later in the nineteenth century. Although Lock Four had scarcely a dozen buildings in the early 1880s, significant growth followed the completion of the Monongahela division of the Pennsylvania Railroad through the community in 1880-1881, the subdivision of part of the Frye farm into building lots in 1885, and additional lots laid out in 1890. More than half of the still extant buildings date from this initial period of rapid growth in the late 1880s and early 1890s, and the village incorporated as the Borough of North Charleroi in July 1894. Growth was further spurred by the industrial development of nearby Charleroi and Monessen, and by 1900 the borough had nearly 500 residents, many employed at the lock and dam, by the railroad, or at a nearby coal mine and glass factory. After the U.S. Government acquired the Monongahela Navigation Company in 1897, the U.S. Army Corps of Engineers began operating Lock No. 4 and the boatyard without tolls, replaced the original stone lock with a concrete structure between about 1913 and 1917, and maintained the boatyard until operations were moved to Neville Island in 1947.