Those changes are pretty complicated and I don't recommend them, because we'd like to improve our calendar in the future and it may be hard to migrate your data then. But if you wish follow these steps:
Edit file modules/CRM/Meeting/MeetingCommon_0.php
find method QFfield_recurrence
add another option - every 4 weeks (with unique integer id) 20 -> __('every 4 weeks')
Find crm_event_get method and add this in the right place (around 650 line nr)
if ($r['recurrence_type']==10) {
$diff = round(($iday-strtotime($r['date']))/(3600*24));
if ($diff<0 || $diff%28!=0) return null;
}
Regards,
Adam
