$\frac{ n \times \left( n-1 \right) \times \left( n-2 \right) \times \left( n-3 \right) \times \left( n-4 \right) ! }{ \left( n-4 \right) ! }$
Cancel out the common factor $\left( n-4 \right) !$$n \times \left( n-1 \right) \times \left( n-2 \right) \times \left( n-3 \right)$
Distribute $n$ through the parentheses$\left( {n}^{2}-n \right) \times \left( n-2 \right) \times \left( n-3 \right)$
Simplify the expression$\left( {n}^{3}-2{n}^{2}-{n}^{2}+2n \right) \times \left( n-3 \right)$
Collect like terms$\left( {n}^{3}-3{n}^{2}+2n \right) \times \left( n-3 \right)$
Simplify the expression${n}^{4}-3{n}^{3}-3{n}^{3}+9{n}^{2}+2{n}^{2}-6n$
Collect like terms${n}^{4}-6{n}^{3}+9{n}^{2}+2{n}^{2}-6n$
Collect like terms${n}^{4}-6{n}^{3}+11{n}^{2}-6n$