\[\sqrt{a^{2} + 8ab + 16b^{2}} =\]
\[= \sqrt{(a + 4b)^{2}} = a + 4b;\]
\[a = 3\frac{3}{7};\ \ b = \frac{1}{7}:\]
\[3\frac{3}{7} + 4 \cdot \frac{1}{7} = 3\frac{3}{7} + \frac{4}{7} = 3\frac{7}{7} = 4.\]