I think downgrading is avoiding the problem with your system. My two VMs have proved that samba4 works. The shared folder error is not a good sign either. Did you try removing all samba shares, removing all shared folders, disable samba, recreate shared folders, enable samba, and recreate samba shares?
I have a feeling that if you downgrade to 3.6.6, it still won't work. If you insist: apt-get install samba=2:3.6.6-6+deb7u10
As for updating, I don't really like aptitude. I always update with omv-update followed by omv-aptclean.