\[\boxed{\mathbf{4.106}}\]
\[\textbf{а)}\ 4 \cdot 7^{3} - 5 \cdot 3^{3} =\]
\[= 4 \cdot 343 - 5 \cdot 27 =\]
\[= 1372 - 135 = 1237\]
\[\textbf{б)}\ \left( 6^{2} - 5^{2} \right)^{3} = (36 - 25)^{3} =\]
\[= 11^{3} = 121 \cdot 11 = 1331\]