The first prototype was designated BM.4b and was fitted with Walter Vega radial engine. The second prototype, flown on 2 April 1928, was designated BM.4d and fitted with the Polish experimental WZ-7 radial engine, then refitted with Le Rhône 9C rotary engine and redesignated BM.4a. The BM.4a became a production variant, because the Polish Air Force had a store of Le Rhône 9C engines. 22 aircraft were ordered and built in 1928–1929 with cowled engines which made it different from all other BM.4s with radial engines.