Got my S4 up and running.
Here's how it looked before I crammed in the HDPlex:
By the way, the HDPlex 250's ATX cables are NOT pin-compatible with the HDPlex 300. The 300 has a single cable and I wanted to use the shorter cable from the 250, so I got a little scared when I put everything together and nothing happened. Switched back to the 300 ATX cable and it posted fine.
The ASRock Z270 ITX Gaming motherboard has been out of stock for over a month now so I said screw it and got the normal ASRock Z270 ITX, which actually has more USB ports, has the M.2 drive on the top side, lower profile heatsinks for the chipset/VRMs, and is a great value at $120 compared to all the various "Gaming" brands from ASRock and others.
I used Aviator snips to sheer the ridges from the 120mm fan bracket, so that I could use the Cryorig 140mm fan and it would lie flush. It fits absolutely perfect, and the rubber grommets on the other side of the fan rest on top of the ram heatsinks, so no vibration at all.
The heatsink is a two-heatpipe, very low profile Thermaltake:
https://www.newegg.com/Product/Product.aspx?Item=N82E16835106152
Using it with the fan gets decent cooling performance at inaudible decibels, even two feet from my head. I'm running it at a fixed RPM.
The CPU I'm using is the 7700k Kaby Lake. What I actually did it slightly underclocked it (it usually runs 4.2ghz with 4.4 boost, but I clocked it to 4.0ghz and undervolted it all the way down to 1.08v (motherboard won't let me go lower). According to HWMonitor, this took the wattage while running Prime95 from 90 watts max while stock, all the way down to 50 watts max while running my configuration. I'm very pleased with this as now I have 55% less wattage for the cooling and less PSU to deal with, with just 4 to 9% less CPU speed.
The only thing making noise in the rig now is the Zotac 1080 Mini and *jeez* is it loud by my standards. The 100mm fan's lowest speed (37% fan speed) is an annoying low hum, and there's no way to hack it to make it quieter. If I tape the fan in place, the thermals hardly rise during benchmarking, but the card becomes near-silent because the 90mm fan is apparently plenty. Great heatsink design I guess! If anything, I'd consider using an adapter to run the 100mm fan off the motherboard so I could get the RPM lower than the minimum.
On the other hand, this 1080 also has pretty bad coil whine. Using Rivatuner to cap the frame limit to 75fps (to match my monitor) reigned it in somewhat, but more intensive games will still really make this card squeal. Googling anecdotes about Zotac suggests they have more coil whine issues than other brands, maybe they just use cheaper inductors. Or maybe the others are fine and I got a dud.
Word of warning, pay special attention if using the S4 Mini with VR. I snagged the headset cable and tipped the case over with a slam. To it's credit, no damage to the case or any components. Wireless VR can't come fast enough...