\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}

\begin{document}
    \tikzstyle{vertex}=[draw,fill=black,circle,minimum size=3pt,inner sep=0pt]
    \begin{tikzpicture}
        \node[vertex] (N-1) at (0,0) {};
        \node[vertex] (N-2) at (0,1) {};
        \node[vertex] (N-3) at (1,0) {};
        \node[vertex] (N-4) at (1,1) {};
    \end{tikzpicture}
\end{document}