A question whether nuclei consisting of four neutrons can exist and how is one of the most important issues to be solved in nuclear physics. In order to resolve this fundamental question, two distinctive experiments were carried out by employing a high-intensity 8He secondary beam. The methods of an exothermic double charge exchange reaction on 4He nuclei and an α knockout reaction on 8He nuclei are introduced and a comparison of the results of the experiments is described. The obtained results exhibit event concentration or a narrow peak just above the four-neutron mass threshold with a broad bump component up to over 60 MeV, which corresponds to continuum states. The observation of the narrow peak provides compelling evidence for the formation of the strongly correlated four-neutron system.