For an HDMI 2.0 mobo, the one that we know of is the Gigabyte Z170X-UD5 TH. Which is in ATX form factor. But there is not yet Iris Pro Desktop part announced by Intel. You could probably stick an i3-6100 in it next month but it would kindda feel like a waste of all those nice graphics ports. Although you could sell the i3 CPU later on and then buy an Iris Pro desktop part. That's assuming Intel doesn't break it's precendent and continues on to make Skylake ones to replace those pre-existing Broadwell parts.
On GPU side there's not much happening until 16nm (Spring / Summer next year). Then really all of the new cards should launch with full support HDMI 2.0 and 4k over DP 1.3 etc. Because there's really no excuse for them not to do that.
However even then, you'd still be better off with Intel IGP for overally lower power / better graphics energy effeciency. Since it's meant for HTPC duties.
Perhaps what you really need is a Skylake version of the existing Intel Broadwell NUCs, and also with the Iris Pro graphics. I don't think they're out yet either. Maybe a Skylake Mac Mini will be release earlier before christmas? We just don't know such things yet. 'soon' could be anytime really.
Alternatively any decent-enough Skylake laptop could also equally perform well such 4k HTPC duties. Then would also benefit from it being a laptop. To take away / do other tasks with.
Another point to be aware of is that Skylake doesn't always fully decode the 4k HVEC VP9 codecs etc. Full fixed-function hardware decoding in the un-slice won't happen until Kaby Lake. So with Skylake some portion of CPU / software decoding may be used, depending exactly which 4k codec(s) are being played back. H264 should be allright though.