newenvironment{myproof}{setcounter{equation}{0}begin{proof}}{end{proof}}

newcounter{savenumber}
newenvironment{myproof}{%
setcounter{savenumber}{value{equation}}setcounter{equation}{0}%
begin{proof}}%
{end{proof}%
setcounter{equation}{value{savenumber}}%
}

%usepackage{amsmath} % preamble
%usepackage{amsthm} % preamble
begin{document}
begin{equation}
    x=1
end{equation}
begin{equation}
    x=2
end{equation}
begin{myproof}
    begin{equation}
        x=3
    end{equation}
end{myproof}
begin{equation}
    x=4
end{equation}
end{document}

newcounter{savenumber}
newenvironment{myproof}{%
renewcommand{theequation}{p.arabic{equation}}% Visual identifier
% renewcommand{theHequation}{p.arabic{equation}}% Uncomment if using hyperref
setcounter{savenumber}{value{equation}}setcounter{equation}{0}%
begin{proof}}%
{end{proof}%
setcounter{equation}{value{savenumber}}
}