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\documentclass[11pt,letterpaper,twoside]{article}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{microtype}
\usepackage[total={6in,10in},left=1.5in,top=0.5in,includehead,includefoot]{geometry}
\usepackage{mathtools}
\usepackage{tabularx}
\begin{document}
\begin{center}
\small
\renewcommand{\arraystretch}{1.5}
\begin{tabular}{|>{\centering\arraybackslash}p{2.5cm}|>{\centering\arraybackslash}p{2.5cm}|>{\centering\arraybackslash}p{2.5cm}|>{\centering\arraybackslash}p{2.5cm}|}
\hline
\multicolumn{4}{|c|}{\normalsize{11. Some table title}}
\\ \hline
\multicolumn{1}{|c|}{\normalsize{Column 1}}
& \multicolumn{1}{c|}{\normalsize{Column 2}}
& \multicolumn{1}{c|}{\normalsize{Column 3$\mathrlap{{}^{(1)}}$}}
& \multicolumn{1}{c|}{\normalsize{Column 4$\mathrlap{{}^{(2)}}$}}
\\ \hline
10 & 1 & $x$ & $y$
\\ \hline
100 & 2 & $u$ & $v$
\\ \hline
1000 & 3 & $p$ & $q$
\\ \hline
\end{tabular}
\end{center}
\vspace{-6pt}
\noindent
\small{(1) Some usefull comments. \newline (2) Another line of useless comments.}
\end{document}