Outline of style I. A. i. a. 1. ...

If you can deal with the four levels provided by the outlines package, then you can combine it with the enumitem package to format the levels:

\documentclass{article}
\usepackage{outlines}
\usepackage{enumitem}
\setenumerate[1]{label=\Roman*.}
\setenumerate[2]{label=\Alph*.}
\setenumerate[3]{label=\roman*.}
\setenumerate[4]{label=\alph*.}
\begin{document}
\begin{outline}[enumerate]
   \1 Level 1 
      \2 Level 2
         \3 Level 3
            \4 Level 4
\end{outline}
\end{document}

Nevermind, I figured it out:

\usepackage{outlines}

\renewcommand{\theenumi}{\Roman{enumi}. }
\renewcommand{\labelenumi}{\theenumi}

\renewcommand{\theenumii}{\Alph{enumii}. }
\renewcommand{\labelenumii}{\theenumii}

\renewcommand{\theenumiii}{\roman{enumiii}. }
\renewcommand{\labelenumiii}{\theenumiii}

\renewcommand{\theenumiv}{\alph{enumiv}) }
\renewcommand{\labelenumiv}{\theenumiv}

\begin{document}