The design also featured a variable-incidence horizontal stabiliser, and the engine mounting was intended to facilitate the ready interchange of different powerplants.

While this work was proceeding, Dobrolyot placed an order with Ukrvozduhput for two photographic survey aircraft, a request that the latter undertook to fill with two specially equipped K-4s.

These machines took priority at the factory, and differed from the passenger version in having a ventral hatch for one or two cameras mounted on the cabin floor, plus a self-contained darkroom.

Piloted by Mikhail Artemevich Snegirev, a K-4 named Червона Украина (Chervona Ukraina - "Red Ukraine") made the trip between 22 and 24 August.

Although the subsequent investigation attributed the crash to pilot error, numerous manufacturing defects were discovered throughout the K-4 fleet, leading to their withdrawal from service shortly thereafter.

Dobrolyot's survey machines commenced operations in August 1928, and were soon joined by around a dozen examples of the airliner version, which were used on routes in Kazakhstan and Central Asia.