\[\boxed{\mathbf{379}.}\]
\[1)\ \left( 15x^{5} + 6x^{4} - 20x^{2} - 8x \right)\ :\]
\[:(3x^{3} - 4)\]
\[2)\ \left( 12x^{5} - 9x^{4} + 8x^{2} - 6x \right)\ :\]
\[:(4x^{2} - 3x)\]
\[3)\ \left( x^{5} + 1 \right)\ :(x + 1)\]
\[4)\ \left( x^{6} - 1 \right)\ :(x - 1)\]