The possible existence of tetra-neutron has been discussed for a long time since the 1960s. In 2016, at RIKEN, the first positive experiment about tetra-neutron system was observed[1]. After, in 2022, a peak in a low energy region by 8He(p,p4He)4n reaction was observed at RIKEN [2]. Inspired of these two experiments, there are several positive theoretical calculations and negative theoretical calculation. In this seminar, I give a talk on recent theoretical situation for tetra-neutron system and related to topics such as super-heavy hydrogen 7H system. [1]K. Kisamori et al., Phys. Rev. Lett. 116, 052501 (2016). [2] M. Duer et al., Nature (London) 606, 678 (2022).