4. Simplify the following expression. \[ 6 a^{4} c+8 a c^{2}-4 a^{4} c-15-10 a c^{2} \] $2 a^{4} c-2 a c^{2}$ $-4 a^{4} c+4 a c^{2}-30$ $2 a^{4} c-2 a c^{2}-15$ $2 a^{4} c+17 a c^{2}-15$

Step 1 ：Rearrange the expression to group the like terms together: \( (6 a^{4} c - 4 a^{4} c) + (8 a c^{2} - 10 a c^{2}) - 15 \)

Step 2 ：Simplify each group of like terms: \( 2 a^{4} c - 2 a c^{2} - 15 \)

Step 3 ：\(\boxed{2 a^{4} c - 2 a c^{2} - 15}\) is the simplified form of the given expression.