> Playing video is very expensive task. Just updating 4K screen at 60 frames per second
Nobody mentioned anything about 60 fps. A ton of video is at 24 or 30 fps.
> requires transferring of 3840×2160×60 ~ 497 million pixels or 1,49 Gbytes per second (if you skip every fourth byte).
You do not need to update the entire screen all the time, you can do partial updates (since very frequently not 100% of the screen changes - it is the most basic fact on which video codecs rely on) and you can "heal" the output over time after partial updates (also important for video playback where you can't guarantee a fast stream).
> I doubt you can do it without a "modern PC" with hardware graphic accelerator.
Why the limitation for hardware graphics accelerators? Hardware accelerators exist for decades now and even hardware video decoders exist for more than a decade. If anything not using those is "not taking advantage of the hardware" that i mentioned.
> Also modern codecs require lot of computations and lot of memory. For example, codecs like VP9 require to buffer up to 8 frames for reference. That would take 8×3840×2160×4 ~ 265 Megabytes of RAM.
Sure, but that is RAM computers also had for more than a decade now - the PC i used in the early/mid-2000s had 2GB of RAM - hell, even the cheapo EeePC netbook from the late 2000s had 1GB of RAM.
> I did some research to see if it is possible to play Youtube video on 8-bit CPUs. What I found is that there is little hope for this. It's better to develop your own video compression format.
Sure, but i never mentioned anything about 8-bit CPUs, YouTube or even existing video compression (or specific framerate for that matter). What i wrote is that you do not need "modern PC power" to do 4K video. You do need more processing power than what would be found in 8-bit CPUs (or at least common 8-bit CPUs you'd find in 80s home computers), but i'm certain you can have a 10 year old PC play 4K video without trying that much. Perhaps even a 15 year old PC with a bit of effort.
FWIW what i had in mind when i wrote that comment was the "8088 Domination" demo which used a custom codec and player to do fullscreen video and audio playback on an original IBM PC and honestly i do not for one second buy the notion that if you can do that on a 40 year old PC you wont be able to have 4K video on a 15 year old PC - if anything i might be too generous here.