\documentclass[tikz]{standalone}
\usepackage{forest}
\makeatletter
\pgfmathdeclarefunction{strrepeat}{2}{%
  \begingroup
    \pgfmathint{#2}\pgfmath@count\pgfmathresult
    \let\pgfmathresult\pgfutil@empty
    \pgfutil@loop
    \ifnum\pgfmath@count>0\relax
      \expandafter\def\expandafter\pgfmathresult\expandafter{\pgfmathresult#1}%
      \advance\pgfmath@count-1\relax
    \pgfutil@repeat
    \pgfmath@smuggleone\pgfmathresult
  \endgroup}
\makeatother
\tikzset{
  Stern Brocot at/.style={at/.pgfmath={
      strcat("([rotate around=180:(!", strrepeat("#1",\SBLevel),")] !",
                                       strrepeat("#1",\SBLevel-1),"2)")}},
  Stern Brocot at*/.style n args={3}{
    at/.pgfmath={strcat("(!",strrepeat("#1",#2),"-| SB@#3)")},
    append after command/.expanded={\ifnum#2<\SBLevel(\noexpand\tikzlastnode)
      edge[densely dotted] (SB@#3@\the\numexpr\SBLLoop+1\relax)\fi}}}
\begin{document}
\begin{forest}
  Stern Brocot*/.style n args={2}{
    content=$\frac{#1}{#2}$,
    edge=densely dotted,
    if={level()<\SBLevel}{append={[,Stern Brocot*={#1}{#2}]}}{}},
  Stern Brocot/.style n args={5}{
    /utils/exec=\edef\SBLevel{#5},@Stern Brocot={#1}{#2}{#3}{#4}},
  @Stern Brocot/.style n args={4}{
    /utils/exec=\edef\SBTop{\number\numexpr#1+#3\relax}%
                \edef\SBBottom{\number\numexpr#2+#4\relax},
    content/.expanded=$\frac{\SBTop}{\SBBottom}$,
    if/.expanded={level()<\SBLevel}{% true
      append={[,@Stern Brocot={#1}{#2}{\SBTop}{\SBBottom}]},
      append={[,Stern Brocot*={\SBTop}{\SBBottom}]},
      append={[,@Stern Brocot={\SBTop}{\SBBottom}{#3}{#4}]}
    }{}}% false (empty)
[,Stern Brocot={0}{1}{1}{0}{4}]
%
\coordinate[Stern Brocot at=1] (SB@left) coordinate[Stern Brocot at=3] (SB@right);
\foreach \SBLLoop in {\SBLevel, ..., 0}
  \path node[Stern Brocot at*={1}{\SBLLoop}{left}]  (SB@left@\SBLLoop)  {$\frac01$}
        node[Stern Brocot at*={3}{\SBLLoop}{right}] (SB@right@\SBLLoop) {$\frac10$};
\end{forest}
\end{document}