I've posted about this on several other forums already, so I'll just go over some of the basic beats.
There are far too many questions regarding this thing to count it as the coolest thing ever. Hyperkin has a history of making extremely flawed clones, has horrible customer service and is very slow to implement revisions that fix problems, and when they do their fixes create other problems. Damned if they do damned if they don't.
The biggest questions for me are
Just what is being fed through HDMI? the Genesis and SNES had RGB encoders which technically can be adapter to support a 240p component signal, which can then be adapted to HDMI. 240p might not sound like much, but this is pretty much what you'd get out of arcade machines back in the day and much better than anything a vintage TV can produce.
GBA is a question, since the video output was intended for a 2.6" LCD, not a tv at all. The GB Player can output 480p, but that's tied with the GameCube hardware.
Just what will they do with the NES side? Will it simply be a composite signal being fed through HDMI as Hyperkin fed a composite signal through the S-Video cable of the RetorN3? Will it be some sort of new NOAC that supports HD resolutions? Will it be software based like the AtGames Genesis portable devices?
GBA support is already possible through various GBA clones, but GBA clones are illegal because the patients of the GBA are far from being expired. If Hyperkin releases this thing with GBA support, they run the serious risk of being sued by Nintendo. That aside, how will GBA games control? Will they be full screen or be windowed like the GB Player? Doubtful, but will it play GB/GBC games?
These and many other questions loom over the RetorN4 like a dark cloud. I'm excited for it's potential, but there needs to be a lot more information provided and given Hyperkin's abysmal history they need to prove themselves king of clones, which they are currently not.