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% Insert the following commented line before \begin{document}:
% \usepackage{multirow}
\begin{tabular}{|l|l|l|l|} \hline
\multicolumn{4}{|l|}{Merged columns} \\ \hline
\multirow{2}{*}{Merged rows} & B2 & C2 & D2 \\ \cline{2-4}
& B3 & \multicolumn{2}{l|}{\multirow{2}{*}{Merged columns and rows}} \\ \cline{1-2}
A4 & B4 & \multicolumn{2}{l|}{} \\ \hline
\end{tabular}