\[\boxed{\mathbf{321.}}\]
\[S^{2} = 23 - 4^{2} = 23 - 16 = 7;\]
\[S = \sqrt{7} \approx 2,65.\]
\[\textbf{а)}\ \lbrack 4 - 2,65;4 + 2,65\rbrack =\]
\[= \lbrack 1,35;6,65\rbrack.\]
\[\textbf{б)}\ 2,\ 3,\ 4.\]
\[\textbf{в)}\ 1.\]
\[\textbf{г)}\ 7,9.\]