  We study $qq \bar q \bar q$-type four-quark (4Q) systems in SU(3)$_c$
anisotropic quenched lattice QCD, using the $O(a)$-improved Wilson (clover)
fermion at $\beta=5.75$ on $12^3 \times 96$ with renormalized anisotropy
$a_s/a_t=4$.For comparison, we first investigate the lowest $q\bar q$ scalar
meson from the connected diagram and find its large mass of about 1.32GeV after
chiral extrapolation, and thus the lowest $q\bar q$ scalar meson corresponds to
$f_0(1370)$.We investigate the lowest 4Q state in the spatially periodic
boundary condition, and find that it is just a two-pion scattering state, as is
expected. To examine spatially-localized 4Q resonances, we use the Hybrid
Boundary Condition (HBC) method, where anti-periodic and periodic boundary
conditions are imposed on quarks and antiquarks, respectively. By applying HBC
on a finite-volume lattice, the threshold of the two-meson scattering state is
raised up, while the mass of a compact 4Q resonance is almost unchanged.In HBC,
the lowest 4Q state appears slightly below the two-meson threshold. To clarify
the nature of the 4Q system, we apply the Maximum Entropy Method (MEM) for the
4Q correlator and obtain the spectral function of the 4Q system.From the
combination analysis of MEM with HBC, we finally conclude that the 4Q system
appears as a two-pion scattering state and there is no spatially-localized 4Q
resonance in the quark-mass region of $m_s< m_q <2m_s$.
