AG Group failover and quorum
The configuration described in the original topic is a bit strange. There are exists two resource groups SYSTEM (CNO, CNO_VIP1, CNO_VIP2, FSW) and AG (LISTENER, LISTENER_VIP1, LISTENER_VIP2, AG).

These two resource groups can be owned by different WSFC nodes. For example SYSTEM is owned by node from DC2 and AG is owned by node from DC1. In this case, if the link between DC1 and DC2 is broken:


Then DC1 will have only 1 vote (NODE1)
Then DC2 will have only 2 votes (NODE3 + FSW)


Accordingly to the quorum configuration DC2 part will alive (due to majority of votes) and DC1 will die (due to minority of votes). But as soon as synchronization between DC1 and DC2 is asynchronous there won't be automatic failover to DC2.

So you will end up with the situation when DC2 is alive but can not handle DB workload, because AG will wait manual intervention (allow_data_loss).

To avoid such situation, you need to assign votes like that

DC1:


NODE1: 1
NODE2: 1


ASYNC SYNCHRONIZATION

DC2


NODE3: 0
NODE4: 0


In that configuration AG will fail over to the NODE2 (DC1).

You'd better to explain the purpose of DC2. Is it disaster recovery DC or you need it for read scale or both?