Plane: updated release notes for 4.0.4

ArduPlane/release-notes.txt

@@ -1,5 +1,5 @@
-Release 4.0.4beta1, 2nd February 2020
--------------------------------------
+Release 4.0.4, 16th February 2020
+---------------------------------
 
 This release includes a significant number of changes from 4.0.3. Key
 changes are:
@@ -13,6 +13,8 @@ changes are:
  - fixed a bug in restoring gains in QAUTOTUNE that could cause the
    aircraft to be unstable on leaving QAUTOTUNE mode
 
+ - fixed stack size of ftp thread
+
 The Copter-4.0 re-sync brings in quite a few structural changes. The
 main user visible changes are: