Error: Extra alignment tab has been changed to \cr. \end{split}

Question

Using this code I get the error: "Extra alignment tab has been changed to \cr. \end{split}". Why so?

\documentclass{article}
\usepackage{amsmath}
\begin{document}
 \begin{equation}
        \notag
        \begin{split}
    Ws & \leftrightarrow I^2(s, \varphi_{W}) \
      I^2(s, \varphi_{W}) & \leftrightarrow I^3(s, \varphi_{W}, \varphi_{I^2})\
        I^3(s, \varphi_{W}, \varphi_{I^2}) &  \leftrightarrow   I^4(s, \varphi_{W}, \varphi_{I^2}, \varphi_{I^3})
    & \ellipsesline
    \end{split}
    \end{equation}
\end{document}