Lock 4 is located in Fall River, between Lake Thomas and Fletchers Lake. Although it has partially collapsed, most of the stone blocks remain visible. This lock is similar in constructed to the restored Lock 1, except it is constructed of both granite and quartzite stones. This stone is believed to be the same stone used during the first construction period of the canal from 1826 to 1831.
While the Shubenacadie Canal was operating during the 1860s, the water levels in both Lake Thomas and Fletchers Lake would have been significantly higher. This can be seen by looking at the height of the water in the channel leading up to the lock and comparing it to the height of the lock. A nearby stone dam helped to keep the water level consistent. Remains of this dam are still visible today.