Add asterisk after labels in enumerate

\documentclass[a4paper]{article}
\newenvironment{modenumerate}
  {\enumerate\setupmodenumerate}
  {\endenumerate}

\newif\ifmoditem
\newcommand{\setupmodenumerate}{%
  \global\moditemfalse
  \let\origmakelabel\makelabel
  \def\moditem##1{\global\moditemtrue\def\mesymbol{##1}\item}%
  \def\makelabel##1{%
    \origmakelabel{##1\ifmoditem\rlap{\mesymbol}\fi\enspace}%
    \global\moditemfalse}%
}

\begin{document}
\begin{modenumerate}
\item uno
\item due
\moditem{*} tre
\item\label{four} quattro
\moditem{*}[\hfill\ref{four}.] quattro
\end{modenumerate}
\end{document}

\begin{enumerate}

\item One

\item Two

\item $\!\!\!\!{^*}$ Three

\item Four

\end{enumerate}

does the trick! Easy.

\! moves the next text a tad to the left, use as many or few as to your preference.