Filming of Outlander season 4 hasn’t started, as of July 2017. However, planning is well under way.
At present we have interesting information about one significant Outlander location, Cambusbarron. Recent reports state that Starz have been negotiating with Stirlingshire Council for the use of local sites, Murrays Wood and Murrayshall Quarry. It seems that they are seeking to make significant use of the sites, including providing access for vehicles and temporary homes. For those who have read the fourth Outlander book ‘Drums of Autumn’, the likely use of this area may be quite apparent.
NOTE : the above link goes to a page that contains potential spoilers!