Thanks folks
The BIOSTAR board looks nice. Pity the price is $30+ higher than the MSI and the ASRock boards.
And I would be concerned about heat and the mobo able to deliver power with 4 gpus side-by-side like Phils pic is showing
Don't even bother doing that if you are not living at a place with extreme cold.
Reference/blower style cards help a lot, since they blow the air AWAY from the cards instead of just right around them.
Per readings elsewhere *ALL* of the Z170 boards seem to have some serious limitations past 4 or 5 GPUs - that chipset doesn't support many PCI-E lanes to transfer data on (relies almost entirely on the ones on the CPU itself) and some of them end up assigned to stuff like M.2 SSD slots and the like.
BTW - I finally got around to doing some tweeking on my my current 3 card 1070 builds, currenty pulling 86-88 MH/s on ETH running around 460 watts at the wall with a Seasonic X850 gold PS *WITH* the GPU on the A10 working MooWrapper - a lower-power CPU/APU would probably drop the system draw 30-50 watts.
Stock BIOS, no undervolt, +550 memory -200 core, 75% power limit in Afterburner - and seems like there's still some room for improvement.
Down side is the cost of the 1070s....
[edit] was playing with a USB-drive based Win10 version, lost over 10% of the hashrate with no other changes. THAT experiment quickly got reverted [/edit]
That BIOSTAR board seems to go on sale at $120 or so every 2-3 months for a bit.