  Many preceding works have shown in $^{11}$Li the presence of the halo
structure comprised of the weakly bound two neutrons around $^9$Li, and it is
intriguing to see how this halo structure changes when another $^9$Li
approaches. In this study, we introduce a four-body model for $^{20}$C with two
$^9$Li clusters and two valence neutrons. The recent development of the
antisymmetrized quasi cluster model (AQCM) makes it possible to generate
$jj$-coupling shell-model wave functions from $\alpha$ cluster models. Here,
$jj$-coupling shell model wave function of $^9$Li is regarded as a cluster,
which corresponds to the subclosure configuration of $p_{3/2}$ for the
neutrons, and we discuss how the two neutrons connect two $^9$Li clusters.
Until now, most of the clusters in the conventional models have been limited to
the closures of the three-dimensional harmonic oscillators, such as $^4$He,
$^{16}$O, and $^{40}$Ca; however, owing to AQCM, it is feasible to utilize the
$jj$-coupling shell model wave functions as plural subsystems quite easily. The
appearance of a rotational band structure with a cluster structure around the
four-body threshold energy is discussed.
