The S.4. Trainer was a further development from the Fokker S.3., also a 2 persons trainer.
The S.4 was the standars trainer for the Dutch Airforce in the twenties. A lot of famous Dutch pilots had their first lessons in it.
The S.4 had a aircooled engine inthe beginning, because these types of engines in the 110-130 Hp class were easily to obtain in that period
then liquidcooled engines.
The Dutch Airforce bougt 30 S.4's, all in the beginning with 125Hp Le Rhone Oberursul engines. In 1926/1927 these engines were replaced by more powerfull 148 Hp Armstrong Siddeley Mongoose.