You are totally misunderstanding 420 vs 444, dude. Look it up, 420 is exactly half the frame size as 444 at the same bit depth.
You can count it for yourself.
444 YCbCr is equivalent in resolution to RGB, right? And uses the same frame size, right? Meaning Y = 8 bits, Cb = 8 bits, Cr = 8 bits, same exact frame size as RGB 8-bits per channel.
For 1080p resolution:
422 uses full Luma res, which is 1080p, but half chroma (810x1080). Since there are two chroma channels, Cb and Cr, the size of each of those is reduced by half, since the resolution is halved. What does that give us in terms of savings? 2/3rds. 1 = Luma, + 1/2 + 1/2 = 2. RGB = 3
420 uses full Luma (1080), and 1/4 for each chroma (so, 810x540).
what's 1 + 1/4 + 1/4 ? 1.5
What's 1.5 / 3?
50%
If your frame size is reduced by half, you can fit twice as many in the same pipe per second. This is fairly straightforward. Look it up if you don't believe me.
Anything that can do 444 at a given resolution and refresh rate, should be able to do 420 at double the refresh rate, if the panel can be refreshed that fast. In terms of bandwidth.
Imagine you have 3 really fat kids that barely fit into the back seat of your SUV. How many small kids, 1/2 the size of the fat ones, can you fit in the same back seat? The answer is.....? SIX. That's double. Same thing.
"It's not correct to say that 4K / 120 using 420 is exactly half the bandwidth as 444. It's 1/4 the colour info but the detail res is the same."
I didn't say that. I said that 4K / 120 at 420 is the SAME as 4K / 60 / 444 (8 bits per channel) in terms of bandwidth. That was my whole point. You could easily refresh a 120hz panel with a 120fps input at UHD if you're content with sticking to 8 bits per channel, and using 420 instead of 444.
UHD / 60 / 444 requires 18 gbps HDMI 2.0 ports. UHD / 60 / 420 can be sent over HDMI 1.4, which maxes out at 10gbps or so. Therefore, UHD / 120 / 420 can be sent over the same 18gbps input ports as UHD / 60 / 444 signals.
This is a fact. NVidia did exactly that to get their older cards to be able to play games at UHD 60 because they don't have the new 19gbps ports. Because 420 is half the size as 444, they were able to increase from UHD / 30 / 444 to UHD / 60 / 420 with a simple driver update.