I can't say for certain, but it sounds like Google has removed support for YouTube from older players. The 4210 is still supported by Roku, but not by all channel providers. Why they would do that, I can't say. the 4210 is still a capable player, but perhaps they simply drew a line in the sand. Since it's appearing on your other devices, that means it's attached to your account, so it will appear on all supported devices.
I know it's a bit of work, but you might try a factory reset. That would clear any possible corrupt code in the channel, and if YouTube is still supported on your player by Google it will be added back in when you link the player back on your account. I've had to do that once or twice over the years, including devices other than Roku.
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