[iMac Pro] Thunderbolt 2, 3, G-Drive, etc.

So the Pro has 4 Thunderbold 3 (USC-C) inputs.

I have 3 g-drives.

1. 2TB - USB 3.0 / 2.0, Firewire 800/ 400
2. 4TB - Thunderbolt and USB 3.0
3. 6TB - Thunderbolt 3

So the 2TB, I know I have no choice but to go with USB.

But the 4TB is also connected via USB and I want to connect that to Thunderbolt, in the hopes of a faster connection.

The 6TB is as I want it.

Any idea what I need to purchase (adaptor, etc.) to get the 4TB to use the Thunderbolt 3 port?

Or should I look into chaining these, one into the other?

Any advice is appreciated.

Thanks.