-- Send a message to the console
function msg(m)
    reaper.ShowConsoleMsg(tostring(m) .. "\n")
end

-- START HERE vvvvvvv

-- get list of selected tracks and make string of GUIDs
local sel_tracks = ''
for i = 0, reaper.CountSelectedTracks(0)-1 do
    sel_tracks = sel_tracks .. reaper.GetTrackGUID(reaper.GetSelectedTrack(0, i))
end
-- store selected tracks to rpp
reaper.SetProjExtState(0, '4PointCut', 'src_tracks', sel_tracks)