[Tex/LaTex] Mix full-width and half-width figures on a single page

Question

I'm using the IEEEconf class and I would like to put 4 graphics in a single page.

Two of them are very large and will occupy two columns, while two of them are small and occupy a single column.

How can I obtain the following result?

Doing the following will result in all the figures being one under the other:

\documentclass{IEEEconf}
\usepackage{graphicx}

\begin{document}
\begin{figure*}[t!]
\begin{center}
\includegraphics[width=\linewidth]{example-image}
\end{center}
\caption{ Empty }
\label{fig:1}
\end{figure*}

\begin{figure*}[t!]
\begin{center}
\includegraphics[width=\linewidth]{example-image-c}
\end{center}
\caption{ Empty }
\label{fig:2}
\end{figure*}

\begin{figure}[t!]
\begin{center}
\includegraphics[width=\linewidth]{example-image-a}
\end{center}
\caption{ Empty }
\label{fig:3}
\end{figure}

\begin{figure}[t!]
\begin{center}
\includegraphics[width=\linewidth]{example-image-b}
\end{center}
\caption{ Empty }
\label{fig:4}
\end{figure}
\end{document}

Answer 1

Here's another solution. It differs from Guilherme's solution only in the way the third and fourth graphs are sized and positioned. Observe that the heights of the graphs are set to a value smaller than \0.333\textheight as it's necessary to leave some space for the captions.

\documentclass{IEEEconf}
\usepackage{graphicx} % for "\includegraphics" macro

\begin{document}
\begin{figure*}

\includegraphics[width=\linewidth,height=0.28\textheight]{example-image}
\caption{ Empty }
\label{fig:1}

\bigskip

\includegraphics[width=\linewidth,height=0.28\textheight]{example-image-a}
\caption{ Empty }
\label{fig:2}

\bigskip

\begin{minipage}{\columnwidth}
\includegraphics[width=\linewidth,height=0.28\textheight]{example-image-b}
\caption{ Empty }
\label{fig:3}
\end{minipage}
\hfill  % maximize the space between the minipages
\begin{minipage}{\columnwidth}
\includegraphics[width=\linewidth,height=0.28\textheight]{example-image-c}
\caption{ Empty }
\label{fig:4}
\end{minipage}
\end{figure*}

\end{document}