[1] The X4 was a low-wing monoplane with a tricycle landing gear and powered by a 116 hp (87 kW) Textron Lycoming engine.
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.
[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] 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.
[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.