\tikzstyle{input_neuron}=[circle,draw=red!50,fill=red!10,thick,minimum size=6mm]
\begin{tikzpicture}[->,thick]
  \node [input_neuron,fill=gray!20] (neuron1) at (0,4) {\small $x_1$};
  \node [input_neuron,fill=red!20] (neuron2) at (1.5,4) {\small $x_2$};
  \node [input_neuron,fill=green!20] (neuron3) at (3,4) {\small $x_3$};
  \node [input_neuron,fill=blue!20] (neuron4) at (4.5,4) {\small $x_4$};

  \draw[->,thick,red!50] (neuron1) -- (neuron2);

  \foreach \x in {1,...,2}
      \draw[->,thick,green!50] (neuron\x) [out=-30,in=-150] to (neuron3);

  \foreach \x in {1,...,3}
      \draw[->,thick,blue!50] (neuron\x) [out=30,in=150] to (neuron4);
      % \draw[->,thick, bend left] (neuron\x) -- (neuron4);

\end{tikzpicture}