Center block on frame with Beamer

If the shortened block is put inside a minipage, you can center it easily

\documentclass{beamer}
\usetheme{Warsaw}

\newenvironment<>{varblock}[2][.9\textwidth]{%
    \setlength{\textwidth}{#1}
    \begin{actionenv}#3%
        \def\insertblocktitle{#2}%
        \par%
        \usebeamertemplate{block begin}}
    {\par%
        \usebeamertemplate{block end}%
    \end{actionenv}}

\begin{document}
    \begin{frame}
        \begin{center}
            \begin{minipage}{4cm}
                \begin{varblock}[4cm]{New block}
                    Variable width, here 4cm
                \end{varblock}
            \end{minipage}
        \end{center}
    \end{frame}
\end{document}

If you use minipage as indicated by samcarter, you can use block and omit the varblock definition since minipage already allows for reducing the width of the block:

\documentclass{beamer}
\usetheme{Warsaw}

\begin{document}
    \begin{frame}
        \begin{center}
            \begin{minipage}{4cm}
                \begin{block}{New block}
                    Variable width, here 4cm
                \end{block}
            \end{minipage}
        \end{center}
    \end{frame}
\end{document}