How to write between two matrix lines

I want to write dots between two lines of a matrix. How can I do this?

I need this output:

with a single line of dots in the fifth column.

• Welcome to TeX.SX! Please help us help you and add a minimal working example (MWE) that illustrates your problem. Reproducing the problem and finding out what the issue is will be much easier when we see compilable code, starting with \documentclass{...} and ending with \end{document}. – BambOo Oct 9 '18 at 21:10

Nest a matrix in the pmatrix:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

$\begin{pmatrix} \begin{matrix} 1 & 2 & 3 & 4 \\ 1 & n & n-1 & n-2 \end{matrix} & \cdots \end{pmatrix}$

\end{document}


You can do that with a simple \multirow:

\documentclass{book}
\usepackage{amsmath, multirow}

\begin{document}

$\begin{pmatrix} 1 & 2 & 3 & 4 & \multirow{1.5}{*}{\!{\dots}} \\ 1 & n & n-1 & n-2 \end{pmatrix}$

\end{document}


Not necessarily the best approach, because it requires a \vcenter or, alternately, a \raisebox, but it is a different way to look at it.

\documentclass{article}
\usepackage{tabstackengine}
\setstacktabbedgap{2ex}
\begin{document}
$\def\stacktype{L} \setstackgap{L}{1.2\normalbaselineskip} \left(\vcenter{\hbox{\Matrixstack{ \stackanchor{1}{1}& \stackanchor{2}{n}& \stackanchor{3}{n-1}& \stackanchor{4}{n-2}& \dots }}}\right)$
$\setstackgap{L}{1.2\normalbaselineskip} \parenMatrixstack{1 & 2 & 3& 4& \\ 1 & n & n-1 & n-2 & \raisebox{.66\dimexpr\Lstackgap}{\dots}}$
\end{document}