local nof_markers = 0
local id = -1

-- check if marker already exists
_, nof_markers, _ = reaper.CountProjectMarkers(0)
for i = 0, nof_markers - 1 do
    local name_tmp = ''
    local isregion_tmp = false
    local id_tmp = 0
    _, isregion_tmp, _, _, name_tmp, id_tmp = reaper.EnumProjectMarkers(i)
    if not isregion then
        if name_tmp == 'DST-IN_4Pcut' then
            id = id_tmp
            break
        end
    end
end

if id >= 0 then
    -- edit existing marker
    reaper.SetProjectMarker(id, false, reaper.GetCursorPosition(), 0, 'DST-IN_4Pcut')
else
    -- create new marker
    reaper.AddProjectMarker2(0, false, reaper.GetCursorPosition(), 0, 'DST-IN_4Pcut', -1, reaper.ColorToNative(0, 255, 0)|0x1000000)
end