[Tex/LaTex] Arrows and label for equivalent resistance with Circuitikz

Question

I have the circuit in black. But I would like to add the red part (label and arrows, no matter the color) using Circuitikz.

Here is the code I'm using:

  \begin{circuitikz}[american, scale = 0.7, transform shape]\shorthandoff{<>}
  \draw 
  (0,0) to[R, l = $R_1$] (0,3)
  to [short] (2,3)
  to [R, l=$R_2$] (2,0)


  (2,3) to[R, l = $R_3$] (5,3)
  to [R, l=$R_4$] (5,0)
  to [short] (0,0)

  (5,3) to[short, -o] (7,3)
  (5,0) to[short, -o] (7,0)

  (7,3.3) node {a}
  (7,-.3) node {b}
  ;
  \end{circuitikz}

Answer 1

Try (using TikZ for red arrow with text):

\documentclass[12pt,border=3mm]{standalone}
\usepackage{circuitikz}
\usetikzlibrary{arrows}
%---------------------------------------------------------------%
        \begin{document}
\begin{circuitikz}[american, scale = 0.7, transform shape]
  \draw
  (0,0) to[R, l = $R_1$] (0,3)
  to [short] (2,3)
  to [R, l=$R_2$] (2,0)

  (2,3) to[R, l = $R_3$] (5,3)

  to [R, l=$R_4$] (5,0)
  to [short] (0,0)

  (5,3) to[short, -o] (7,3) node[above]  (a) {a}
  (5,0) to[short, -o] (7,0) node[below]  (b) {b}
  ;
  \draw[>=angle 90,<->,red,% borrowed from TikZ, gives red arrow ...
        shorten <=1mm, shorten >=1mm] (a) to node [fill=white] {RTh} (b);
  \end{circuitikz}
\end{document}