My setup: MacMini with 1TB internal, 2TB external USB. If I added a 4TB disk to my Airport Extreme as a disk on there USB port and use it as a Time Capsule Drive, will Time Machine on my MacMini also backup my 2TB external drive to it?
Yes, as long as you don't exclude it in the optional TM settings.
Thanks
Just to clarify. External drives are automatically added to the exclude list with Time Machine. So you need to open Time Machine prefs and go to the Option button and remove the external from the excluded list. Then it will be backed up.