startPat = {
  \improvisationOn
  \omit Stem
}
stopPat = {
  \improvisationOff
  \undo \omit Stem
}

\new Voice \with {
  \consists Pitch_squash_engraver
} {
  c'4 d' e' f' |
  \startPat
  4 4 4 4 |
  \stopPat
  f'4 e' d' c'
}