Originally designed as a 4-seat ATL aircraft, and at the time called the ATL II or ATL.FAR23, it was later intended to become a long-term replacement for the DR.400 series of aircraft.[2] After Pierre Robin sold his company, the name was changed to X4, X for experimental and 4 for 4-seater; the design was also changed from the ATL's V tail to a more conventional cruciform type. The fuselage was fibreglass and epoxy in a Nomex sandwich, which allowed more fluid curves, and was generally triangular in cross-section, like the Me 262.[3] The landing gear was from a DR.400, and the forward-tilting canopy from an ATL.[4][5] The wings were wood and fabric, like those of a standard DR.400, but of constant dihedral; the wooden construction allowed modifications to be made quickly and cheaply.[4] The rudder and ailerons were of metal construction.[4]
It first flew on 25 February 1991, with Robin head of development Daniel Müller at the controls; whilst designed as a four-seater, only the front two seats were installed with the rear being taken up with test equipment. The airframe was used to test various wing profiles, especially laminar flow; the feasibility of producing a composite-material aircraft; and to test new systems (e.g. rod rather than cable controls).[4]
Testing at Saint-Cyr showed a slight advantage to the X4 when compared to the equivalent DR.400/120, despite, according to Müller, its 'tired'[6] engine. For example, optimisation of the cowling reduced engine cooling drag by 20%, or 5% of global drag; in total there was a 25 km/h gain in cruise speed.[7] However, the improved performance came at the cost of a non-benign stall unsuitable for a training aircraft.[6] According to Robin and Besse, the airframe was capable of eventually being a whole series up to 4+2 seat configuration with correspondingly larger engines, and be a potential competitor to the Cirrus SR22. The new owners of Avions Robin were not interested in the design, and it was ultimately scrapped.[6]