\begin{Schunk}
\begin{Sinput}
 X4mat <- X[ , 4, drop = FALSE]
 is.matrix(X4mat)
\end{Sinput}
\begin{Soutput}
[1] TRUE
\end{Soutput}
\begin{Sinput}
 is.vector(X4mat)
\end{Sinput}
\begin{Soutput}
[1] FALSE
\end{Soutput}
\end{Schunk}
