  In neutron-rich $^8$He, we study the soft dipole resonance, which is regarded
as a dipole oscillation of four valence neutrons against the $^4$He core, and
its effect on the low-energy electric dipole strength with a
$^4$He+$n$+$n$+$n$+$n$ five-body cluster model. This work is an extended study
of an earlier letter [T. Myo and K. Kat\=o, Phys. Rev. C106, L021302 (2022)].
The five-body unbound $1^-$ states of $^8$He are obtained with the complex
energy eigenvalues by using the complex scaling method and the dipole strength
is calculated in terms of the complex-scaled Green's function. Two kinds of the
dominant excitation modes are confirmed in the dipole strength below 20 MeV of
the excitation energy. The strengths below 10 MeV are exhausted by the
$^7$He+$n$ channel, which sequentially decays to $^6$He+$n$+$n$. Above 10 MeV,
the strengths arise from the soft dipole mode of four neutrons ($4n$)
oscillating against the $^4$He core. We further explore the possibility of the
soft dipole resonance for this state by carefully searching for the resonance
pole and finally predict the corresponding resonance with the excitation energy
of 14 MeV and the decay width of 21 MeV. The soft dipole resonance exhausts
about half of the dipole strength in the relative motion between the $^4$He
core and $4n$.
