  Nonmesonic weak decays of the A=4, and 5 hypernuclei are studied. The short
range parts of the hyperon-nucleon weak interactions are described by the
direct quark (DQ) weak transition potential, while the longer range
interactions are given by the $\pi$ and $K$ meson exchange processes. Virtual
$\Sigma$ mixings of the coherent type are found to give significant effects on
the decay rates of $^4_{\Lambda}{\rm He}$. A large violation of the $\Delta I =
1/2$ rule is predicted in the J=0 transition amplitudes.
