  We study the manifestly exotic tetraquark D$_{\rm s0}^{++}(cu\bar s \bar d)$
and the scalar tetraquark $f_0(ud \bar u \bar d)$ in SU(3)$_c$ anisotropic
quenched lattice QCD with the $O(a)$-improved Wilson (clover) fermion with
various quark masses including the idealized SU(4)$_f$ case. For $f_0(ud\bar u
\bar d)$ etc., we only consider connected diagrams at the quenched level, i.e.,
the tetraquark $f_0(ud\bar u\bar d)$ is identical with D$_{\rm s0}^{++} (cu\bar
s \bar d)$ in the idealized SU(4)$_f$ case. First, for comparison, we study the
lowest $q\bar q$ scalar meson, and find that it has a large mass of about
1.37GeV after chiral extrapolation, which corresponds to $f_0(1370)$. Second,
we investigate the lowest 4Q state in the spatial periodic boundary condition,
and find that it is just a scattering state of two pseudoscalar mesons, as is
expected. Third, to extract spatially-localized 4Q resonances, we use the
Hybrid Boundary Condition (HBC) method, where anti-periodic and periodic
boundary conditions are imposed on quarks ($c$,$u$) and antiquarks ($\bar
s$,$\bar d$), respectively. By applying the 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, we find a nontrivial 4Q
resonance state about 100 MeV below the two-meson threshold in some quark-mass
region. Its chiral behavior largely differs from a two-meson scattering state.
The scalar tetraquark $f_0(ud\bar u\bar d)$ is found to have the mass of about
1.1GeV after chiral extrapolation, and seems to correspond to $f_0(980)$. Then,
the manifestly exotic tetraquark D$_{\rm s0}^{++}(cu\bar s \bar d)$ would exist
around 1GeV in the idealized SU(4)$_f$ chiral limit. Finally, MEM analysis is
applied to obtain the spectral function of the 4Q system.
