exchange 2013 DAG | Standalone database

You are combing two different concept into one. Think again, what are you trying to achieve.

The autodiscover process works but after that it fails to connect to DB1.

So two things.. 1) is the database actually online (Get-MailboxDatabase DB1 -Status) and 2) what is the AutoDiscoverServiceInternalUri set to (Get-ClientAccessServer | Select Identity, AutoDiscoverServiceInternalUri)

Most cases when a full site fails you need to change some DNS A records, which is usually autodiscover.company.com along with mail.company.com. If your autodiscover namespace internally is the same all around, you might think AutoDiscover is working but its not. You have two options:

1) Setup a separate namespace in each site for AutoDiscover (so the primary site is autodiscover.company.com/autodiscover/autodiscover.xml and the other active site is dr-autodiscover.company.com/autodiscover/autodiscover.xml) or
2) Manually change the A record for Autodiscover to point to the surviving sites CAS or CAS Virtual IP (for load balancing)

As for the send / receive connectors I would need more information on how that is actually configured. In theory you should have a send connector to the internet or smarthost (which then sends to the internet) that has all four CAS within there. If that does fail then you are set, as the other two surviving CAS would still utilize it.

my fault on the applications relay, i hadn't checked anonymous.

If i want to use site 2 for a DR test and the DR test involved killing the WAN link, i could remove svr4 from the DAG and add it again once the WAN link is restored?  DB1 would then mount on SVR4 whilst the WAN link is down?

is the database down when you kill the WAN link? I would imagine so since the cluster manager is going to take all the databases offline due to lack of quorum.

OK.  I should be able to mount the database under the condition that i remove the server from the dag?

great help