(A possible answer.)
Setting a global variable (anyone, not just this one), does not take effect immediately:
If you set it in my.cnf (etc), it would take effect until after a restart
If it is set globally (SET GLOBAL @@... ), it won't take effect until someone logs in -- he then gets that setting. (Caveat: Some globals have no "session" counterpart, so ...
Make it a 4-node Galera Cluster. Have gcache set large enough to handle more than 30 minutes' worth of changes. Then let Galera take care of the rest.
Meanwhile, your clients should not connect just to galera4, but via a proxy to any node. (You may want to tell the proxy that "galera4" is preferred.)
But the real problem comes when you try to ...