subequations: How to continue numbering within subequation?

Question

Let's assume we want to create one subequation including four aligned single equations. The first two single equations should be numbered as 1a and 1b while the third and fourth one should continue the numbering of the previous one, but with new equation number as 2a and 2b.

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Minimum Working Example (MWE):

\documentclass{article}
\usepackage{amsmath}

\begin{document}
    \begin{subequations}
        \begin{align}
            1   &=  1\\
            2   &=  2\\
            3   &=  3\\% from here a new equation number should begin
            4   &=  4
        \end{align}
    \end{subequations}
\end{document}

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Screenshot of the result:

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Questions:

1. How can I automatically continue numbering of the previous equation numbers without cheating around by manually assigning numbers/tags?
2. Would it be possible to increase the vertical space between the equation 1-pair and the equation 2-pair?

Answer 1

I could not find the duplicate, so here is my attempt:

\documentclass{article}
\usepackage{amsmath}
\newcommand\StepSubequations{
  \stepcounter{parentequation}
  \gdef\theparentequation{\arabic{parentequation}}
  \setcounter{equation}{0}
}
\begin{document}
    \begin{subequations}
        \begin{align}
            1   &=  1\\
            2   &=  2\\
            \StepSubequations
            3   &=  3\\% from here a new equation number should begin
            4   &=  4
        \end{align}
      \end{subequations}

      \begin{equation}
        \label{eq:1}
        a=b
      \end{equation}

\end{document}