Two double-line equations need double left brackets

Question

I have four lines of equations. The first two and the second two need to be grouped by a left bracket, and all four need to be grouped by one overall left bracket.

I am using an array within an equation. I can't manage to add the two 'inner' left brackets. The one big left bracket over all four equations is not a problem, which is the following:

\begin{equation}
    \left\{
      \begin{array}{lcl}
        1\\
        2\\
        3\\
        4
      \end{array}
    \right.
\end{equation}

And this seems to work just fine. But when I try the following

\begin{equation}
  \left\{
    \begin{array}{lcl}
      \left\{ 
        \begin{array}{lcl}
        1\\
        2
        \end{array}{lcl}
      \right.
    \end{array}
    \begin{array}{lcl}
      \left\{      
        \begin{array}{lcl}
        3\\
        4
        \end{array}
      \right.
    \end{array}
  \right.
\end{equation}

it doesn't work. I've tried some other changes, including changing to eqnarray, or changing the places of the brackets, but nothing seems to work. Now I am not sure how to solve this.

Answer 1

I think that some information about the actual expressions might be useful to decide which structure(s) would be best to use; in its present form (using arrays for the two subgroups), one could use a cases environment for the outer brace:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation}
\begin{cases}
      \left\{ 
        \begin{array}{lcl}
        1 \\
        2
        \end{array}
      \right. \\
      \left\{      
        \begin{array}{lcl}
        3 \\
        4
        \end{array}
        \right.
\end{cases}
\end{equation}

\end{document}

Here's another option, using a gather environment for the whole structure, and nested arrays:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{gather}
  \left\{
    \begin{array}{@{}l@{}}
      \left\{ 
        \begin{array}{lcl}
        1 \\
        2
        \end{array}
      \right. \\
      \left\{      
        \begin{array}{lcl}
        3 \\
        4
        \end{array}
      \right.
    \end{array}
  \right.
\end{gather}

\end{document}

By the way, your code has some typos like \end{array{lcl}.