The M.F.4 was based on the first aircraft model built by Marinens Flyvebaatfabrikk, the Marinens Flyvebaatfabrikk M.F.1, and was intended to replace the latter on a 1 to 1 basis. Amongst the modifications added to the M.F.4 model was such changes as a monoplane tail section and a shorter wingspan.
Production
The M.F.4 was manufactured at Marinens Flyvebaatsfabrikk's facility in Horten from 1918, the production of the trainer aircraft necessitating an expansion of the aircraft factory and the construction of a new workshop. A total of six aircraft were manufactured between 1918 and 1921. Two of the M.F.4s were constructed in 1918 by converting M.F.1 airframes. The first M.F.4 aircraft built, F.2 , was equipped with a Renault 130 hp V-8 engine until rebuilt with a Scania-Vabis PD engine on 22 July 1918. The other five aircraft of the type built were all powered by the Scania-Vabis engine. The last M.F.4 to be manufactured, F.12A , was first test flown on 7 April 1921.
Service
The type's first flight occurred on 13 March 1918, when F.2 was test flown by the Marinens Flyvebaatfabrikk. The M.F.4 was the first purpose-built trainer aircraft in service with the Royal Norwegian NavyAir Service; the M.F.1s that had been used up until then having to be improvised in order to perform the trainer task. In 1920 the Royal Norwegian Navy Air Service and the civilian air company Det Norske Luftfartrederi cooperated on starting the first experimental civilian passenger/post route in Norway. Initiated by the Norwegian postal service, the route was flown between the port cities of Horten, Kristiania and Kristiansand. When the Supermarine Channelflying boats employed on the route suffered technical malfunctions, other military seaplanes took over several flights. Amongst the aircraft used were the M.F.4. By the early 1920s the M.F.4 aircraft, and the preceding M.F.1, were outdated and unsuited for the service they were supposed to perform. In one accident the M.F.4 flown by the head of the Royal Norwegian Navy Air Service's flying school lost its wings. The replacement for the M.F.4, the Marinens Flyvebaatfabrikk M.F.6, had its first flight on 3 April 1920. Even though outdated, the M.F.4 remained the most flown aircraft in the Royal Norwegian Navy Air Service's inventory until retired in October 1924. During the type's close to six years of service, three of the six aircraft were written off in crashes. No aviators were killed or injured flying the M.F.4. The last of the M.F.4s, F.6 , was removed from service on 2 October 1924, having flown a total of 489 hours and 40 minutes.
