\documentclass{article}

\usepackage[fleqn]{amsmath}

\begin{document}

\begin{flalign*}
  \begin{alignedat}{2}
  f(x) &= (\text{expression 1}) &&+ (\text{expression 2}) \\
       &                        &&- (\text{expression 3}) \\
       &= (\text{expression 4}) &&
  \end{alignedat} &
\end{flalign*}

\end{document}