filter the nth note

[Stefan Thomas]

Dear community,
I would like to do something like the following, but automatically, without
typing all the music again:
allNotes= { c'8 g' e' g' d' g' f' g' c' g' e' g'   }
someNotes = { c' d' c' }
In other words: I would like to filter out every 4th note of allNotes, so
that I could do in combination with changePitch (see
https://github.com/gilles-th/arranger.ly/blob/master/changePitch.ly) the
following:
patI = { c8 r r4 }
someNotes =  { \cPI { \someNotes s } }
Thanks for Your help,
Stefan