I think it's because the default resolution is 4096*2160
Pick nightly image from here: Index of /
4K@60Hz was certainly an issue at some point and I'm not sure if it is fixed in image linked in this topic but it's certainly fixed in nightly image (most of the time I test on 4K TV). Currently it will force 4K@30Hz in this case because I can't locate the issue for enabling high speed link in HDMI driver, but I'm close.