A Micromegas 1 detector with four active layers, serving as prototype for the upgrade of the ATLAS muon spectrometer 2, was designed and constructed in 2014 at CERN and represents the first example of a Micromegas quadruplet ever built. The detector has been realized using the resistive-strip technology and decoupling the amplification mesh from the readout structure. The four readout layers host overall 4096 strips with a pitch of 415μm; two layers have strips running parallel (η in the ATLAS reference system, for measuring the muon bending coordinate) and two layers have inclined strips by ±1.5° angle with respect to the η coordinate in order to provide measurement of the second coordinate. A detector characterization carried out with cosmic muons and under X-ray irradiation is presented with the obtained results. •The Micromegas detector with four readout layers is the first quadruplet ever built.•The characterization of it is valuable for the ATLAS New Small Wheel upgrade.•This is part of the validation procedure for the Micromegas New Small Wheel detectors.