Split a long equation and align each lines of it in LaTeX

Using amsmath package and multline environment splits a long equation but the second line of splitted equation is placed to the right. On the other hand, split (not requiring amsmath package) with & align a splitted equation.

\documentclass{article}
\usepackage{amsmath}
\begin{document}


\begin{multline*}
f(x) = x + 2x + 3x \\
+ 4x
\end{multline*}


\begin{equation}
\begin{split}
f(x) = x + 2x + 3x \\
       &+ 4x
\end{split}
\end{equation}


\begin{equation}
\begin{split}
f(x) = x &+ 2x + 3x \\
         &+ 4x
\end{split}
\end{equation}


\begin{equation}
\begin{split}
f(x) = & x + 2x + 3x \\
       &+ 4x
\end{split}
\end{equation}


\begin{equation}
\begin{split}
f(x) &= x + 2x + 3x \\
     &+ 4x
\end{split}
\end{equation}


\end{document}

The indentation of the first equation using multline is not so good. In the above example, the third pattern (labeled with (2)) may look better than others.

\begin{equation}
\begin{split}
f(x) = x &+ 2x + 3x \\
         &+ 4x
\end{split}
\end{equation}