Postgresql Vacuum and replication
The standby server has to write the WAL that it gets from the primary server, it needs enough disk space for that.

Since DB2 is not only a standby, but also a primary server for DB3 and DB4, it might need to retain WAL for a while (I don't know how you configured that).

Just give all standby servers as much disk space as the primary has, then you should be good. Be generous with disk space!

It could be that extra disk space is used if you use replication slots and a standby cannot keep up, perhaps because of a slow network connection. That could be the cause of your out-of-disk condition.

Remember that DELETE and VACUUM do not free any disk space.