The M4 unit is the deformable mirror providing the E-ELT with adaptive correction of the atmospheric turbulence. The mirror is segmented into 6 petals which are actively shaped by more than 5000 voice-coil actuators. They are controlled in close loop with internal metrology through co-located capacitive position sensors. INAF is involved in the optical calibration and verification of the M4 unit and designed the laboratory optical testbed In this paper, we present a possible auxiliary setup for the mirror calibration once installed at the telescope. The concept implements an on-demand, day-time, optical re-calibration of the mirror, to ensure the years-long term, high accuracy, high precision stability of the internal metrology, beyond the already remarkable intrinsic electronic stability of the M4 unit. The setup exploits the two focii of the quasi-elliptical M3 to create an optical cavity, with the interferometer placed at a Nasmyth focal station of the ELT and a retroreflector (or fiber source) at the M3 short focus to measure the M4 in double (or single) pass. The full monitoring of the M4 optical area allows to: calibrate the actuator influence functions to compute the segments piston tip/tilt commands with high precision; retrieve the flattening command to correct from the low and high order features generated by thermo-mechanical and electrical drifts; compute the phasing command to correct for the segments differential alignment and piston within the requested WF accuracy. The system offers a fast and effective optical maintenance facility for the M4U, without requiring an additional test tower and the mount/dismount down-time of the unit. In this work, we summarize the optical layout and the flattening and segments co-phasing strategy.