Can I backup a Time Capsule to a HDD drive connected to its USB port? I have a 2TB Time Capsule and multiple external HDDs.

I recently bought a 4TB external drive, now I'd like all my files, in all my HDDs to go to the 4TB drive that is connected through USB to the Time Capsule, including its internal drive!
How can I make this happen?

So, is there a solution to backup to a new Time Machine on the 4TB drive that includes the Time Capsule's internal drive, my computers, and two external HDDs?
Time Machine does not include external drives by default —but this can be changed if the external drives are formatted for Mac: click the Options button on the Time Machine preferences and remove the desired external disk from the Exclude this items from backup list so it will be backed up.

Time Machine does not backup network resources.
You could move the contents from your Time Capsule to a local or external HFS USB disk to back them up.