\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}

\begin{tikzpicture}[
roundnode/.style={circle, draw=green!60, fill=green!5, very thick, minimum size=7mm},
squarednode/.style={rectangle, draw=red!60, fill=red!5, very thick, minimum size=5mm},
]
%Nodes
\node[squarednode]      (maintopic)                              {2};
\node[roundnode]        (uppercircle)       [above=of maintopic] {1};
\node[squarednode]      (rightsquare)       [right=of maintopic] {3};
\node[roundnode]        (lowercircle)       [below=of maintopic] {4};

%Lines
\draw[->] (uppercircle.south) -- (maintopic.north);
\draw[->] (maintopic.east) -- (rightsquare.west);
\draw[->] (rightsquare.south) .. controls +(down:7mm) and +(right:7mm) .. (lowercircle.east);

\end{tikzpicture}
\end{document}
