It looks like we have a typo in our Upgrade instructions. Apologies.
When upgrading, the "mvn -U clean package" command MUST be run from the
[dspace-source] directory (NOT from [dspace-source]/dspace/).
Running the command from [dspace-source] ensures that *all* DSpace
modules are recompiled/rebuilt for DSpace 4.0. (Whereas, running the
build from [dspace-source]/dspace/ performs a "quick build", which does
NOT recompile all modules.)