  Tetraneutron ($^4n$) has come back in the limelight, because of recent
observation of a candidate resonant state at RIBF. We propose to investigate
the pion double charge exchange (DCX) reaction, i.e. $^4\mathrm{He}({\pi}^- ,
{\pi}^+)$, as an alternative way to populate tetraneutron. An intense ${\pi}^-$
beam with the kinetic energy of ~850 MeV, much higher than that in past
experiments at LAMPF and TRIUMF, will open up a possibility to improve the
experimental sensitivity of the formation cross section, which will be much
smaller than hitherto known DCX cross sections such as $^9\mathrm{Be}({\pi}^-,
{\pi}^+)^9\mathrm{He}\ (g.s.)$.
