The HDMI spec for 4K is 1.4. The HD621 uses 1.3, so there's a fair chance it won't I would have thought.

However, given the lack of true 4K material, no true 4K players (or format for that matter), the cost of screens big enough for 4K to actually make a difference, etc., etc. I'm not sure I'd see a 4K screen as an "investment". Also I'd have thought that you're not going to see a lot of difference between up scaling in the Oppo and up scaling in the TV anyway, in which case is the HD621 really a bottleneck?

As the still happy owner of a PDP5000EX I'd recommend you just go ahead and get the sound top notch and not worry about increased visual clarity that your eyes couldn't resolve anyway.

Sooloos, Roon, G series (with LPS upgrade), 800 series (with LPS upgrade), HD621, AC200, Prime and PS, DSPs old and new.

Purdeys, Moke, Leica, 'Blad. Only the best.