I think the 4k options now are:

1. Buy a 4k player (e.g Panasonic)which has 2 HDMI outputs and which combined should allow a 4k image with lossless audio via the HD621.

2. Buy one of the new Lumagen 4k video processors (which also has 2 HDMI ouputs) and will do a good job of upscaling standard BRs (expensive option). There aren't many UHD BRs out there so you won't be missing much in the way of native 4k replay.

3. Buy 1 and 2 (even more expensive).

4. Wait.

Main system - Sources (Kodi-HTPC, Vero 4k, FireTV, Sat) into Trinnov Altitude 32 (8-16) (+Roon), Trinnov Amplitude 8 amp & Vivid Audio (Giya G3, V1.5) and REL Stentor III speakers. Others: F80 + Raspberry Pi and other Pi systems.