+If you are upgrading from version 3 or earlier you have two options:
+
+1. adapt your code to the new API (all changes needed are described in [doc/api_changes_v4.md](doc/api_changes_v4.md))
+
+2. use instead the *compatibility layer* which is provided. Instructions and pitfalls described in [doc/api_changes_v4.md](doc/api_changes_v4.md##enabling-compatibility-with-legacy-code)
+
+In any case, read carefully the docs in [doc/api_changes_v4.md](doc/api_changes_v4.md) and report back any undocumented
+issue using GitHub.