Yep, as GMT has written its all pretty straightforward. I have two systems running with HDFury devices (one Vertex2 and one Diva).

All your 4k sources go via HDMI to the Vertex2. The Vertex2 splits into a signal to your 4k display and another to your HD621.

My 861 (and before that G68) just has one 4kSRC input, with the Vertex doing the switching between sources.

Main system: 818v3, 861v6, HD621, Oppo 103D, DSP8000PP, DSP7200SE HC, DSP3200, SGR Sub
Family room: HD621, 861v6, DSP5200 L/C/R
Headphones: Schiit Mjolnir v1, Sennheiser HD800; (office) iFi iDSDPro, Focal Elegia