documentclass[11pt,a4paper]{article}
usepackage[margin=3cm]{geometry}
usepackage[english]{babel}
usepackage{amsmath, amsthm}
begin{document}

begin{proof}
    Hello there good chap! I'll put in another line here just to be sure its clear what I mean.
end{proof}



end{document}

usepackage{etoolbox}
patchcmd{endproof}
  {popQED}
  {parpopQED}
  {}
  {}

documentclass[11pt,a4paper]{article}
usepackage[margin=3cm]{geometry}
usepackage[english]{babel}
usepackage{amsmath, amsthm}

usepackage{etoolbox}
patchcmd{endproof}
  {popQED}
  {parpopQED}
  {}
  {}

begin{document}

begin{proof}
    Hello there good chap! I'll put in another line here just to be sure its clear what I mean.
end{proof}

end{document}

documentclass[a4paper]{article}
usepackage{amsthm}

% lower the symbol and make it zero width
renewcommand{qedsymbol}{raisebox{-baselineskip}{llap{openbox}}}

begin{document}

begin{proof}
This is a proof that this setting is emph{really bad}
end{proof}

end{document}

documentclass{article}
usepackage{amsthm,amsmath,xpatch}
newtheorem{theorem}{Theorem}

% xpatchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
xpatchcmd{qed}{leavevmode}{parnobreakleavevmode}{}{}

begin{document}
begin{proof}
begin{equation}
  f(x) = ax^2 + bx + c
end{equation}
end{proof}

begin{proof}
begin{equation}
  f(x) = ax^2 + bx + c
end{equation}
Some more text.
end{proof}
end{document}