Merging inside align-environment

If this remark line is not part of the math lines, then using intertext{..} is another way for doing this.

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

    \begin{align*}
      lefthandside1 &= righthandside1\\
      lefthandside2 &= righthandside2
      \intertext{This is a remark before the third line.}
      lefthandside3 &= righthandside3\\
      lefthandside4 &= righthandside4
    \end{align*}

\end{document}

Another method is to use \noalign{..}. This will be similar to the \shortintertext.

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

    \begin{align*}
      lefthandside1 &= righthandside1\\
      lefthandside2 &= righthandside2\\
      \noalign{This is a remark before the third line.}
      lefthandside3 &= righthandside3\\
      lefthandside4 &= righthandside4
    \end{align*}

\end{document}

You could use TeX's multispan (inspiration here).

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

    \begin{align*}
      lefthandside1 &= righthandside1\\
      lefthandside2 &= righthandside2\\
      \multispan2{Remark before the third line\hfil}\\
      lefthandside3 &= righthandside3\\
      lefthandside4 &= righthandside4
    \end{align*}

\end{document}

You can use shortintertext of the mathtools-package:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}

    \begin{align*}
      lefthandside1 &= righthandside1\\
      lefthandside2 &= righthandside2\\
      \shortintertext{\hfil This is a remark before the third line.}
      lefthandside3 &= righthandside3\\
      lefthandside4 &= righthandside4
    \end{align*}

\end{document}

If you leave out the \hfil the remark is flushed left.