custom drumstuff, positions of noteheads

[Stefan Thomas]

Dear community,
I want to create a customized drumstaff for four toms.
What I don't understand: in the below quoted snippet the positions of the last three drum notes are the same.
Why is it so and how can I change it?

\version "2.12.2"
%%% four Toms
#(define fourtoms '((hightom default #t  4)
                   (himidtom default #t 1.4)
                    (lowmidtom default #t -1.2)
                     (lowtom default #t -3.8)
                   ))


fourtomStaff = {
  \set DrumStaff.drumStyleTable = #(alist->hash-table fourtoms)
  \override Staff.StaffSymbol #'line-positions = #'(4 1.4  -1.2 -3.8)
  \override Staff.BarLine #'bar-size = #4
}
musik = \drummode { tomh 4 tommh tomml toml }
\score {
  \new DrumStaff { \fourtomStaff \musik  }
}