\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  W_r=\begin{bmatrix}
    0 & r{J_t}/{\mu} & 0 & {g l^3 m^3}/{\mu^2} \\[0.5ex]
    0& {lm}/{\mu} & 0 & {g l^2 m^2 (m+M)}/{\mu^2} \\[0.5ex]
    {J_t}/{\mu} & 0 & {g l^3 m^3}/{\mu^2} & 0\\[0.5ex]
    {lm}/{\mu} & 0 & {g l^2 m^2 (m+M)}/{\mu^2} & 0 \\
  \end{bmatrix}.
\end{equation*}
\end{document}