Make align* number the last equation

What you want is aligned with bottom vertical alignment:

\documentclass{report}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{aligned}[b]
x & =2+2\\
  & =3+1\\
  & =4
\end{aligned}
\end{equation}
\end{document}

It's easier suppressing the first lines. Every line ends up with \\ except the last one. So simple redefine \\ inside the environment align.

\documentclass{report}
\usepackage{amsmath}
\makeatletter
\def\Let@{\def\\{\notag\math@cr}}
\makeatother
\begin{document}
\begin{align}
x & =2+2\\
  & =3+1\\
  & =4
\end{align}
\end{document}

I'm a little surprised no one has gone for the 'simple' solution involving \notag

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align}
x & =2+2\notag\\
  & =3+1\notag\\
  & =4
\end{align}
\end{document}