Multiline box over subset of aligned equations

Question

I have a multilined equation which is aligned as follows: (schematically)

\documentclass[english]{article}
\usepackage{amsmath}
\usepackage{babel}
\begin{document}

\begin{align}
\text{Equ} &\equiv \text{first line of RHS} \;+\nonumber \\
 & \phantom{=}+\;\text{second line of RHS}\nonumber \\
 & =\text{third line of RHS} \; +\nonumber \\
 & \phantom{=} +\; \text{fourth line of RHS} 
\end{align}

\end{document}

What I'd like to achieve is a multiline box over only the third and fourth lines as follows:

I have tried using the \boxed{} function but this wouldn't work over multiple lines. Note: I would prefer not to box the entire four lines in this case.

Is this possible and, if so, how is it done?

Edit: (subsequent to answer by Bernard) I am having 2 difficulties implementing the pstricks solution: 1) The numbering seems to start over outside the postscript environment. 2) Equation references \eqref{} don't seem to work with \label{} inside the postscript. See output and MWE below:

\documentclass[english, svgnames]{article}
\usepackage{mathtools}
\usepackage{babel}
\usepackage{pst-node}
\usepackage{auto-pst-pdf}

\begin{document}

\begin{postscript}
\begin{align}
\text{Equ} & \equiv \text{first line of RHS} +{} \\
                         & =+\;\text{second line of RHS} \\
\pnode[-0.6em, 2.5ex]{A} & =\text{third line of RHS} + {} \\
                         & = +\; \text{fourth line of RHS}   \pnode[0.8em, -1.5ex]{B}   \label{Equ1}
\end{align}
\psframe[linejoin=1, linecolor=black](A)(B)
\end{postscript}

\begin{align}
\text{Equ2} &= \text{first line of RHS}\\
&= \text {second line of RHS}\label{Equ2}
\end{align}

Reference to Equ1: \eqref{Equ1}.  Reference to Equ2: \eqref{Equ2}.

\end{document} 

Answer 1

It is easy with pstricks: place an empty node at the beginning of the third row, another empty node at the end of the fourth row, and draw the frame with opposite vertices these two nodes. The optional argument of \pnode is an offset.

This code can be compiled with pdflatex if you set the --enable-write18 switch for MiKTeX, -shell-escape for TeX Live and MacTeX.

Alternatively, do not load auto-pst-pdf, remove the postscript environment, and compile with xelatex.

\documentclass[english, svgnames]{article}
\usepackage{mathtools}
\usepackage{babel}
\usepackage{pst-node}
\usepackage{auto-pst-pdf}
\begin{document}
\begin{postscript}
  \begin{align}%
    \text{Equ} & ≡ \text{first line of RHS} +{} \
                             & =+;\text{second line of RHS} \
    \pnode[-0.6em, 2.5ex]{A} & =\text{third line of RHS} + {} \
                             & = +; \text{fourth line of RHS} \pnode[0.8em, -1.5ex]{B}
  \end{align}
  \psframelinejoin=1, linecolor=Coral(B)
\end{postscript}
\end{document} 

Added : there seems to be problems with cross-references, which work well only in this version of the code, for use with xelatex:

\documentclass[english]{article}
 \usepackage{mathtools}
\usepackage[showframe]{geometry}
 \usepackage{babel}
\usepackage{pst-node}%
\usepackage{cleveref}
\begin{document}
\begin{align}%
 \text{Equ} & ≡ \text{first line of RHS} +{} \label{Equ1}\
 & =+;\text{second line of RHS} \
\pnode[-0.6em, 2.5ex]{A}& =\text{third line of RHS} + {} \
 & = +; \text{fourth line of RHS}\label{Equ4}\pnode[0.8em, -1.5ex]{B}
 \psframelinejoin=1(B)
 \end{align}}
 \eqref{Equ1} and \cref{Equ4}
\end{document}

Answer 2

Shamelessly borrowed from Werner at https://tex.stackexchange.com/a/358675/4427

\documentclass{article}
\usepackage{amsmath}

\usepackage{tikz}
\usetikzlibrary{fit,tikzmark}

\begin{document}

\begin{align}
\text{Equ} &\equiv \text{first line of RHS} \nonumber \\
 &\quad + \text{second line of RHS} \nonumber \\
 \tikzmark{start1} &=\text{third line of RHS} \nonumber \\
 &\quad + \text{fourth line of RHS} \tikzmark{end1}
\end{align}
\begin{tikzpicture}[remember picture,overlay]
  \coordinate (start) at ([yshift=1.7ex]pic cs:start1);
  \coordinate (end) at ([yshift=-0.3ex]pic cs:end1);
  \node[inner sep=2pt,draw=red,fit=(start) (end)] {};
\end{tikzpicture}

\end{document}