The fuselage was significantly longer (3.3 m or 10 ft 10 in) partly because of the decision to seat pilot and co-pilot side by side and further forward, clear of the wing leading edge. From there, forward to the nose the top of the fuselage was now horizontal, rather than falling away downwards as on the civil aircraft. This change, giving a poorer forward view resulted from fitting the longer inline Lorraine 12 Ds in the bomber rather than the short Salmson radials of the Latécoère 4. A rectangular radiator immediately behind the two-bladed propeller provided cooling, replacing the external radiators of the transport. There was a serious dispute between the State and Latécoère about the engine choice; briefly, the former wished all BN4 machines to use more powerful Renault engines but failed to have them available, whereas Latécoère preferred the Lorraines on the grounds of a better power-to-weight ratio. The outer engines of the bomber were similarly mounted to those of the Latécoère 4 with bracing to the wing roots and fuselage, but in deeper and flatter sided cowlings which extended downward almost to the wing. These outer engines had similar rectangular radiators and propellers to that of the central one.
