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The design of the interferometric test of the adaptive M4 Unit of E-ELT, a deformable six petals 2.4 m mirror, will be described. The actual baseline follows a macro-stitching approach, where each segment is separately flattened and co-phased to the other petals. The optical test setup for the single shell consists in a Newtonian system, with a 1.5 m parabolic mirror as main collimator. A 0.6 m reference flat mirror is foreseen to verify the alignment of the interferometric cavity. A Demonstration Prototype of the final M4 Unit, a 222 actuators, two shells deformable mirror, has been produced by Microgate and A.D.S. International. Results of the optical measurement campaign performed in INAF on the prototype mirror are reported.