[Opm] Release 2020.04 rc4

[Joakim Hove]

OK;

now rc4 versions of the release packages are out. Hopefully this will be 
the last round of rc releases. Since the rc4 packages were created at 
least this: https://github.com/OPM/opm-simulators/pull/2606 is a 
candidate which could have been backported to the release; however I 
have decided that this particular bug is not sufficient to hold back the 
release - sorry; on the other hand I would be more positive to make a 
bugfix release e.g. around june 1.st.


Joakim