Do you mean after Cliff?
I can imagine that you'd use the second HDMI output on a theoretical 4k Radiance to feed the HD621 at 1080p whilst the primary goes to your 4k capable display. You would lose the M comms switching (which is why I intend to stick a Mini3D after my 421) but would hopefully avoid any problems with 4K going through a 'legacy' device.

Mark
G68, baby Genelecs, Humax Foxsat, Oppo+Vanity93, Toshiba HD-A35, Pioneer CLD-925 + Meridian 519, various other random bits and bobs
Got a black G08.2 you want to sell?