Bold horizontal and vertical lines in Longtable

Question

I am using Longtable. For the following MWE, how can I make the third vertical line and the second horizontal line thicker than the other lines?

\documentclass[12pt, twoside]{article} 
\usepackage{longtable}
\begin{document}
\begin{longtable}[c]{|l|c|c|c|}
\hline
 & Col 2 & Col 3 & Col 4 \\ \hline
\endfirsthead
Row 2 & a & b & c \\ \hline
Row 3 & x & y & z \\ \hline
\end{longtable}
\end{document}

Answer 1

Instead of longtable, I suggest the tabularray package:

\documentclass[12pt, twoside]{article} 
\usepackage{tabularray}


\begin{document}
\begin{longtblr}{
  colspec={|l|c|[3pt]c|c|}
}
\hline
 & Col 2 & Col 3 & Col 4 \\ \hline[3pt]
Row 2 & a & b & c \\ \hline
Row 3 & x & y & z \\ \hline
\end{longtblr}
\end{document}

(For more details on how to do longtables with tabularray, see chapter 4 of the package documentation)