having an engine at 3.6.5 and following

it is not clear in my opinion what to do with 3.6 repos if I want to upgrade to 4.0

In fact if you strictly follow what indicated, you should only run 

yum update "ovirt-engine-setup*"

but this eventually shows 3.6.7 packages.

And I presume that also with 3.6 repos in places and already being in 3.6.7 the command above doesn't bring you to 4.0

Also, you cannot run something like

yum update ovirt-release

because they are different packages:

In my flow I ran

yum install http://resources.ovirt.org/pub/yum-repo/ovirt-release40.rpm

and then disable ovrt-3.6, otherwise 

yum update "ovirt-engine-setup*"

anyway brings in some packages from 3.6 repos (3.6.7 I think) that seems to be identical with 4.0 ones.

Gianluca