Reading the spec for HDMI 1.4 I'd guess that it's a hardware issue, not software.

However, it might not actually matter. If you read the spec for 1.4 you'll see that there appears not to be any increase in bandwidth between it and 1.3. 1.4 adds an ARC (Audio Return Channel) which is possibly the last thing an HD621 needs, an Ethernet channel (see comment on ARC), an Automotive Connection System (again, if this is something to do with cars), 3D over HDMI (HD621 already transmits this), etc.

So it may well be that despite never being 1.4 compliant it doesn't actually matter as far as the HD621 is concerned. So long as it can strip off the audio and pass-through sufficient bandwidth of video it can happily stay 1.3 and still function. 1.3 requires 340MHz of bandwidth (same as 1.4). This is sufficient to pass 2160p30. Until you actually get a 4K signal into one it may be difficult to tell what limitations the unit has.

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.