we have 2 exchange server running as mailbox and 2 exchange running HCS.

I'm getting this error:

database on this server (Mailbox 2) exceeded the configured maximum database size and service recovery was not attempted. Please increase the value of this registry key on the server hosting the active database copy before attempting to mount this database.       For more details about the failure, consult the Event log on the server for other storage and "MSExchangeIS" events. 

• Edited by one2ool 9 hours 5 minutes ago

DAG configure on Exchange Mailbox servers 

This article explains what to do: https://technet.microsoft.com/en-us/library/bb232092%28v=exchg.141%29.aspx?f=255&MSPPError=-2147217396

Once you have the database mounted again, you should look at creating more databases and migrating mailboxes to it. This will make management easier in the long run, and is worth the effort.