O.Thetford, P.Gray German Aircraft of the First World War (Putnam)

Pfalz D Type
   This aircraft was a biplane version of the E V monoplane. It was to the Walfisch format, with deep gap-filling fuselage and car-type radiator at the nose. Although not confirmed, the engine is thought to have been 100 h.p. Mercedes D I. No other data available.

W.Green, G.Swanborough The Complete Book of Fighters

PFALZ D.4 Germany

   Designated D.4 by its manufacturer, the Pfalz Flugzeug-Werke’s first essay in the single-seat fighting biplane category, flown in the summer of 1916, was powered by a 105 hp Daimler D I six-cylinder water-cooled engine with a car-type radiator. An unequal-span, slightly-staggered single-bay biplane, the D.4 featured an exceptionally deep fuselage eliminating the normal cabane structure and afforded an extremely poor forward view for the pilot who was unable to see the horizon in level flight and was virtually blind for take-off and landing. Flight testing revealed poor handling characteristics, the rudder being blanketed by the deep fuselage and directional stability being totally inadequate. A fixed vertical fin was added at an early stage, but this failed to ameliorate the more serious shortcomings, development being discontinued and Pfalz building the LFG Roland D I (and subsequently the D II). No data relating to the D.4 have survived.

J.Herris Pfalz Aircraft of WWI (A Centennial Perspective on Great War Airplanes 5)

Pfalz D 4

   The uninspired D 4 (factory designation) was the first Pfalz attempt to produce a biplane fighter. The D 4 fuselage was very similar to that of the Pfalz E.V, but its depth was increased to fill the gap between the wings in a manner similar to the Roland C.II. Nevertheless, the narrow wing gap must have caused interference in the airflow between the wings, limiting performance. The D 4 was powered by a 150 hp Benz Bz.III engine. At least two configurations were seen, one with fixed fin and pilot's headrest, and one without. Flown in the summer of 1916, the D 4 was not placed in production as contemporary biplane fighters from Halberstadt and Albatros were far superior.