In an effort to produce an aircraft with the best possible forward visibility for the observer, Gallaudet developed the D-2 into the D-4, which placed the observer in the nose, followed by the pilot, with the propeller attached to a ring that circled the fuselage behind the wings. The four blades spun around the fuselage just behind the pilot.
Power for the D-4 was a 360-hp Liberty L-12 engine buried within the fuselage, just behind the pilot. A large pontoon was mounted below the fuselage, and small outboard floats were mounted near the tips of the swept-back wings.
Two D-4 were built for the Navy (A2653, A2654). The prototype crashed during flight testing on 19 July 1918, but the second was accepted by the U.S. Navy. No further aircraft were built, as Gallaudet turned their attention to constructing Curtiss HS-2L flying boats.
Engine: 1 × Liberty L-12, 360 hp ( kW) each