Since the Beebox's claim to fame is the dual-channel memory, just what sort of bandwidth is on offer? I employed AIDA64's bandwidth test, the results of which you can see above. The latency is pretty high, but the bandwidth is pretty decent for a SoC.
Unfortunately, I don't have a 4K native panel at home to test with, but I was able to run the provided test software to see some 4K performance, albeit downscaled.
The ASRock Beebox managed a solid 30 FPS throughout most clips I tried, including YouTube 4K playback using Google Chrome. YouTube 4K playback does require a bit over 7 Mb/s, so if your network is capable, so is the ASRock Beebox. It's not 60 FPS, but given the price here, I can't really complain about getting "only" 30 FPS.