RN314 moving the whole (4-disk) volume to one disk?

There is no official way to do that, but here is a way that will work.  The only thing is, that you will have no redundancy while you do it, so you really should have a backup.  And if you have a backup, you could just start fresh with the new drive and restore data from backup.

But, here is the process, which will only work if your volume is RAID:

Disable XRAID.

Remove one drive, making the volume "degraded" (not redundant).

Insert the new drive and create a new volume on it.

Copy all the files to the new volume.  That includes temporarily copying any personal folder content somewhere else.

Uninstall apps (because the next step will make them not work)

EXPORT your old main volume.  This will cause the NAS to make your new volume the primary one.

Re-install any apps.

Make sure every user that has a home folder that contained data has logged on, so their home folder is re-created.  The command line tool mkhomedir_helper can be used from SSH instead if you are adept at SSH.

Move files from temporary location back to home folders.

Note that I had you EXPORT the old volume, not DESTROY it.  Having so done, you could return all those drives to the unit (with power off) and it will re-import (as a secondary volume) in case you find you missed something.  But it also means your orignal main one can no longer be installed as a main volume.

Since you had to ask this, I am assuming you do not have a backup of your data.  If you are relying on RAID to keep your data safe, you could some day find that the NAS volume is inaccessible and you either have to spend a bundle of money for data recovery or your data is just plain gone.  You really need to consider backup unless everything on your NAS is already backup (so you could get it back from the original sources, even if tedious).