The F.4 and F.5 were designed in parallel with the 890-hp (664-kW) Isotta-Fraschini Asso 121 inline and 870-hp (649-kW) Fiat A.74 radial engines respectively. It was then decided to fit the more powerful Daimler-Benz DB 601A in the F.4, and the delay thus caused meant that the F.5 was the first to fly, an event that took place early in 1939.
The F.4 had traditional mixed construction, with welded-steel-tube fuselage with light alloy skin and wooden wings. Landing gear and flaps were hydraulic, and ailerons were arranged to droop with extension of the flaps. Tailplanes had variable incidence for trimming.
The F.4 finally flew in late 1940 with the DB 601A, and it was then decided to concentrate on an improved F.6 with the DB 605A.