Welcome to LuLa.
Start by skimming this thread and the Medium article I started (gotta get back and add to that).
http://forum.luminous-landscape.com/index.php?topic=116168.0As to your specifics:
- Don't worry as much about bootable clones for your OS - leverage Time Machine to do that (continuously)
- If you want to do a quarterly clone, keep it onsite ( I like SuperDuper )
- How much of the 4tb will be used initially? How many gigs do you average per year?
- Are you looking at the desktop or laptop 4tb external drives? (if laptop, you'll want a powered USB3 hub)
- You don't say what size TimeMachine drive you're looking at - 5 to 8tb would all work (and the 8tb's are dropping in price).
USB3 is fine for platter drives, you'd need some external SSDs or more disks to make the TB really shine.
So your day in/day out use would have 2 external drives connected via USB. Main data drive (usb1) has continuous backup via Time Machine (usb2) and an online provider (I like BackBlaze). Every X amount of time you do a Chronosync of your data drive (usb1) to your offsite copy (usb3), and if you're feeling frisky you clone off your boot SSD for sanity before OS upgrades.
Shy of a really bad series of events, or ignoring error messages, you would be covered well.
-Joe