Removing horizontal space in align environment?

I am assuming that your real equation contains more thrilling entries than a, b, c, and d. If that's the case, you may be better off employing a flexible array environment.

\documentclass{article}
\usepackage{array} % for '\newcolumntype' macro
\newcolumntype{C}{>{{}}c<{{}}} % for binary and relational operators
\newcolumntype{L}{>{\displaystyle}L} % automatic display-style math mode, left-aligned
\begin{document}
\[
\setlength\arraycolsep{0pt}
\renewcommand\arraystretch{1.2} % to emulate vert. spacing of 'align' env.
\begin{array}{LCLCLCLl} % the final column is a "dummy" column
  a &=& b &+& c &+& d\\
  b &=& b &+& c &+& d\\
  c &=& b &+& c &+& d\\
  aoeu &=& \multicolumn{6}{l}{\mbox{something long here}}
\end{array}
\]
\end{document}

As I don't know the real stuff that will be in your alignat* environment, I kept the three alignment columns. A solution consists in putting the r.h.s. of the last row in an \rlap command, which makes it have 0 width. In case this r.h.s. were a maths formula, mathtools defines a \mathrlap command, which has the same effect in math mode:

\documentclass[11pt]{article}

\usepackage[utf8]{inputenc}
\usepackage{fourier}
\usepackage{mathtools}

\begin{document}

\begin{alignat*}{3}
  a &= b &{}+{}& c &{}+{}& d\\
  b &= b &{}+{}& c &{}+{}& d\\
  c &= b &{}+{}& c &{}+{}& d\\
  aoeu &= \rlap{something long here}\\
 a & =\mathrlap{\frac{-b\pm\sqrt{b^2-4ac}}{2a}}
\end{alignat*}

\end{document}