No new features, just one bug fix.
This bug only occurred within an undo operation, (fortunately) most people never use the undo feature. Links to composite glyphs were updated just a bit too late. This caused serious access violations and the loss of all unsaved data.
This release solves this bug.
http://www.high-logic.com/prerelease/fcp406.zip
Unzip it and copy it to your existing fcp4 folder.