If I understand aright you had to manually install the release package for 7.4 beta:
yum install http://packages.nethserver.org/nethserver/7.4.1708/updates/x86_64/Packages/nethserver-release-7-3.1.ge457d21.ns7.noarch.rpm
EDIT!!!
Now I have found the following:
It’s the same for 7.5. But why we get the updates automatically at the software center?