You can always go forward. So WF4 can open WF3 but you can't open a native WF4 with WF3. There is a free plug-in however that allows you to open a WF4 with WF3 or WF2. It freezes all of the features created in the newer versions and allows you to work on it with the older version. When you open it back up in the newer version you will get all of the features again.