Centering a value of a matrix
A simple way would be to use the nicematrix
package.
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\[
\begin{pNiceArray}{CCCC}
1 & 0 & 0 & 0 \\
0 & 1 & 0 & 0 \\
0 & 0 & \Block{2-2}{R} & \\
0 & 0 & &
\end{pNiceArray}
\]
\end{document}
Raise it into place:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{pmatrix}
1 & 0 & 0 & 0 \\
0 & 1 & 0 & 0 \\
0 & 0 \\
0 & 0 & \multicolumn{2}{c}{\raisebox{.5\normalbaselineskip}[0pt][0pt]{$R$}}
\end{pmatrix}
\]
\end{document}
A solution that keeps the existing pmatrix
setup and combines \multicolumn
and \multirow
directives:
\documentclass{article}
\usepackage{amsmath,multirow}
\begin{document}
\[
\begin{pmatrix}
1 & 0 & 0 & 0 \\
0 & 1 & 0 & 0 \\
0 & 0 & \multicolumn{2}{c}{\multirow{2}{*}{$R$}} \\
0 & 0
\end{pmatrix}
\]
\end{document}
In short, all you need to do is replace \multicolumn{2}{l}{\multirow{R}}
in row 3 of your code with \multicolumn{2}{c}{\multirow{2}{*}{$R$}}
and delete \multicolumn{2}{l}{}
in row 4 of the pmatrix
.