(a)
Since, $f( - x) = - f(x)$ and $f(x + 2) = f(x)$
$\therefore\ \ \ f(x) = f(0)$ and $f( - 2) = f( - 2 + 2) = f(0)$
Now, $f(0) = f( - 2) = - f(2) = - f(0)$
$\Rightarrow 2f(0) = 0\ \ \Rightarrow \ \ \ \ f(0) = 0$
$\therefore\ \ \ f(4) = f(2) = f(0) = 0$