Depending on size and scope of the project, you’re probably best off using the one SSD drive, and leaving the 4TB for other types of storage, such as original camera cards and such.
The other consideration too is connection type. An SSD is good and fast, but if you’re not filling up the connection pipe (i.e., Thunderbolt, Firewire, USB 2, etc.). In other words, depending on the connection type, both harddrives may give you the same performance regardless of how fast they are.
The bigger consideration might be how quickly cache/render files balloon on you. I believe by default Premiere stores cache on the boot drive, which is probably fine in most cases, but at 512 GB might dwindle quickly if you don’t stay on top of keeping it cleaned out.
Splitting across two drives might be a consideration for managing storage, but I don’t think there’s an inherent speed bump–at least not one that you’d notice. You’re probably just fine off the SSD alone. But admit, I could be wrong about this. One the plus side, it would mean only hauling around one HD.
Same for FCPX (not sure about Resolve), though for FCPX keeping the cache external to the Library might help you manage that storage better than keeping it internal, but still more a preference based on workflow and organization. Don’t think there’s speed gains either way.
For balance, I found these threads. Though old, one advocates the multi-drive setup, and the other notes SSD is fast enough for everything (dependent on connection type):
https://forums.adobe.com/thread/2099128
https://forums.creativecow.net/thread/2/1039174
— Joby.