In general the RX 460 supports multiple outputs, so it should be able to drive both the BenQ and the Viewsonic (I assume OS X supports multiple displays, so that should not be an issue). You might need to update your OS X to a newer version to get RX 460 support, and you will need the automate-eGPU.sh script, but I'll let the Mac crowd chime in to elaborate on this. Connect the 4K external monitor via DisplayPort from the RX 460 for video output.120W (12v, 10A, 5.5x2.5mm barrel plug) AC/DC power brick to replace the adapter used by the Thunder2.AMD RX 460 2GB: A version with no external power connector and one small enough to fit the Thunder2.

Since you don't game I went for the simplest setup. If this is not an option: Your simplest (and pretty much cheapest) eGPU option to drive a 4K display. Just use a DisplayPort connection from said port to the 4K display. Does your system have a DisplayPort1.2 port in addition to the Thunderbolt1 ports? That would solve your quandary cheaper and faster.

Imac 27 late 2013 external display