\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{xcolor}
\usepackage{tikz}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{4}{lllc}}
\toprule
\textbf{Value B} & \textbf{Group C} &  & \textbf{Value A} \\
\midrule
\multirow[t]{2}{*}{a} & x & \textbf{only} & 1 \\
 & y & \textbf{only} & 3 \\
\multirow[t]{2}{*}{b} & x & \textbf{only} & 2 \\
 & y & \textbf{only} & 4 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}