As long as you see all of your databases attached to the same MySQL server (true most likely for most customers), then the hostnames are all interchangeable.
So just use the 'add hostname' button and add hostnames for B, C and D then edit the appropriate wp-config.php with the new hostname for each domain. Then each will have a hostname associated with it's own domain.
You do need to do this before domain A expires.
What the hostname does is allow use of a NAME used to locate the IP address of the database. The reason you can use one single hostname for all 3 domains currently is they are all on the same database server and thus all have the same IP address.
Since your sorting this out in anticipation of a domain expiring I would just go ahead and create 3 new hostname something like mysql.domainB.com mysql.domainC.com mysql.domainD.com. All of those hostnames be interchangeable with this existing, then just update each domain to it's own hostname.