According to heise/c't magazine[1] (highly respected German tech publication), the Vega-based APUs support HDMI 2.0 even if the motherboards only advertise HDMI 1.4 support, as the pins are directly routed from the APU package. DisplayPort 1.2 works for 4K@60Hz as well of course. I suppose there's a risk the motherboard traces might be too low quality to carry HDMI2 signal speeds on some boards; hopefully board makers will revalidate and update specs for their boards. On the other hand, it may be worth waiting the 5-8 weeks until 400 series chipset boards come out which will presumably be designed with Ryzen G in mind.

[1] This is mentioned in their Ryzen G motherboard review/roundup (paywall): https://www.heise.de/ct/ausgabe/2018-5-AM4-Mainboards-fuer-A... - also the review of the CPUs: https://www.heise.de/ct/ausgabe/2018-5-Ryzen-Prozessoren-mit...