  We investigate the multineutron excitations of neutron-rich $^8$He in the
electric dipole strength using a $^4$He+$n$+$n$+$n$+$n$ five-body model.
Many-body unbound states of $^8$He are obtained in the complex scaling method.
We found that the different excitation modes coexist in the dipole strength
below the excitation energy of 20 MeV. The strength below 10 MeV comes from the
$^7$He+$n$ channel, indicating the sequential breakup of $^8$He via the $^7$He
resonance to $^6$He+$n$+$n$. Above 10 MeV, the strength is obtained from
many-body continuum states with strong configuration mixings, suggesting a new
collective motion of four neutrons with the dipole oscillation against $^4$He.
