documentclass[tikz, margin=3mm]{standalone}
usepackage{forest}
usetikzlibrary{shapes.geometric, }

begin{document}
forestset{
 rect/.style = {rectangle split,
                rectangle split horizontal,
                rectangle split parts=4,
                draw},
          }% end of forestset

begin{forest}
  for tree={
      rect,
      %
           }
[nodepart{one} A
% nodepart{two} B,
% nodepart{three} C,
 nodepart{four} $D$
]
end{forest}
end{document}

! Undefined control sequence.
pgfkeyscurrentkey ->nodepart
                               {four} $D$