A.Weyl Fokker: The Creative Years (Putnam)

Fokker was still keen to build seaplanes. The Admiralty paid well, and their officials were much less fussy than those of the Army, in which the growing influence of engineering specialists was creating difficulties for Fokker. He had an advantage in the shape of the Schwerin; lake of his competitors, only the Flugzeugbau Friedrichshafen (a Zeppelin subsidiary) had facilities for trying out new seaplanes close to the factory.
   The conversion of the M.7 into a twin-float seaplane seemed feasible. The wing loading would be too high for good take-off performance, however, and a re-design was needed.
   With the permission of the Reichsmarineamt, one of the rejected M.7s was converted and modified. It was fitted with two-bay wings of greatly increased span; the interplane struts were inclined inwards at such an angle as to provide internal wing bracing bays of equal length. The upper wing had long extensions, the landing wires and warp-return cables being supported by the usual inverted-vee king-posts.
   The floats were those of the unsuccessful W.2; they were of wooden construction, plywood-covered, and had obviously been designed by somebody connected with naval aviation. These floats, more than a year old, were connected to the M.7-type fuselage by a strut system that was more robust than that of the W.2. The new seaplane floated tail-high, but a small tail-float was retained as a precaution.
   The completed aircraft was given the type number W.3. It proved to be unsatisfactory. A few trials by Fokker indicated its reluctance to leave the water, although it carried virtually no load. It is uncertain whether the naval authorities discouraged development or Fokker still lacked the patience to make modifications and develop systematically. At any rate the prototype was dismantled, reconverted into a standard M.7 landplane, and supplied for training purposes to the Marine-Landflieger Abteilung at Johannisthal.

J.Herris Fokker Aircraft of WWI. Vol.1: Spinne - M.10 & Watercraft (A Centennial Perspective on Great War Airplanes 51)

Fokker W.4
  
  The Fokker W.4 was a floatplane conversion of the M.7 two-seat reconnaissance plane. Fokker re-used the floats from the W.2 (and W.3) for the conversion together with an M.7 built for the Navy. To provide more lift to compensate for the additional weight of the floats, new, much larger two-bay wings were fitted. The engine was an 80 hp Oberursel U.0.
  Despite the larger wings, the modest engine power meant the W.4 could not take off from water. The W.4 was re-converted to an M.7 land plane and sent to the Marine-Land-Flieger Abteilung at Johannistal where it was used as a trainer.

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

Fokker W 4
   This aircraft was a seaplane development of the M 7 which had also been used by the German Navy. Only a few short flights were made during March 1915. Engine was 80 h.p. Oberursel U O rotary.