  Room temperature optically detected magnetic resonance experiments on spin
3/2 Silicon vacancies in 4H-SiC are reported. The $m_s=+1/2\leftrightarrow
-1/2$ transition is accessed using a two microwave frequency excitation
protocol. The ratio of the Rabi frequencies of the $+3/2 \leftrightarrow +1/2$
and $+1/2\leftrightarrow -1/2$ transitions is measured to be $(0.901\pm
0.013)$. The deviation from $\sqrt{3}/2$ is attributed to small difference in
g-factor for different magnetic dipole transitions. Whereas a spin-1/2 system
is characterized by a single spin lifetime $T_1$, we experimentally demonstrate
that the spin 3/2 system has three distinct relaxation modes that can be
preferentially excited and detected. The measured relaxation times are
$(0.41\pm 0.02) T_{slow}=T_d= (3.3\pm 0.5)T_{fast} $. This differs from the
values of $ T_p/3 =T_d= 2T_f $ expected for pure dipole ($T_p$), quadrupole
($T_d$), and octupole ($T_f$) relaxation modes, respectively, and implies
admixing of the slow dipole and fast octupole relaxation modes.
