  We study the spatial structure of four valence neutrons in the ground state
of $^8$He and $^{18}$C nuclei using a core+4$n$ model. For this purpose, we
employ a density-dependent contact interaction among the valence neutrons, and
solve the five-body Hamiltonian in the Hartree-Fock-Bogoliubov (HFB)
approximation. We show that two neutrons with the coupled spin of $S$=0 exhibit
a strong dineutron correlation around the surface of these nuclei, whereas the
correlation between the two dineutrons is much weaker. Our calculation
indicates that the probability of the (1p$_{3/2})^4$ and [(1p$_{3/2})^2$
(p$_{1/2})^2$] configurations in the ground state wave function of $^8$He
nucleus is 34.9% and 23.7%, respectively. This is consistent with the recent
experimental finding with the $^8$He($p,t)^6$He reaction, that is, the ground
state wave function of $^8$He deviates significantly from the pure
(1p$_{3/2})^4$ structure.
