I don't doubt this is going to be good SFF case, but I see it's another mid-size-SFF that focus on supporting multiple modes rather than be optimized for one specific purpose. As result it got fat 14.5mm (30mm thicker than A4). So it's a compact case, but again will not be very portable.
I was hoping to have something like the Ghost S1 but at about 124~130mm thick, to make it easier to fit in backpacks (for example 'Manfrotto Manhattan mover-50'), and to keep it lightweight (which Dan A4 did great at 1.25kg, but Ghost S1 did so poorly at about 3kg without the TopHats needed for water-cooling).
Ie given that Dan already have great small mostly air-cooled case with the A4, the C4 would be great to focus on water-cooling while keeping the slim profile and lightweight construction.
Imo the target parameters should be around 125~130mm which will fit fine 240mm rads (which are typically 280mm long incl. the ports).
The basic body (imagine a Ghost S1 w/o any hats) should be as tall to barely fit the itx-mobo with minimal extra space for cables (& later tubes) ie about 185mm.
Just for comparison I was seriously considering buying A4 and modding it with rads on top (and bottom) with 240mm rads, but its height of 200mm was over the limit. The height of a "Hat" module containing a 240mm rad would be about 55mm tall, and it you want to fit 2 of those (to make it fully water-cooled and overclockable) it means that you'll need to add at least 110mm to the main body. The limit for how tall such case should be is imo at 300mm (again for easier choice of backpacks), which means that a bottom+body+top rads should be something like at most: 55mm+190mm+55mm (ideally the main body should be few mm shorter to allow for small tolerances).
The length of the case is a bit easier in regards to fitting in backpack, because most of them can fit at least 400mm (and many get closer to 500mm).
The minimal length imo should be about 350mm (so that it has little extra space for separate pump like EK SPC-60 for example, probably with just a top instead of reservoir to save on space). But length can be easily increased to 400mm to allow for more PSU mounting orientations, and extra space for cables, tubes & perhaps small or flat reservoir. Ideally length should be adjustable, perhaps with extendable rails. Now ideally all dimensions should be extendable, but that'll be more complex to design, rather than fix 2 of them and only add rails for the 3rd dimension.
Regarding weight if you can keep the main body under 1.5kg (from the A4 it's obviously possible), and the additional Top/Bottom "hats" about 0.5kg each it'll give still portable (like backpackable) sff pcat least if watercooled with full-aluminium loop like EK-FluidGaming for example. The target for fully built pc like this incl. two hats (Top & bottom) ie with 2x240mm rads, in a single loop with single pump, should be around 7kg and under 13L.
Here's a rough example for my idea, with the case frame made of 8 pieces of 60x20x2mm aluminium extruded profiles with necessary length and holes (minus the side-panels and top-n-bottom mesh filters)
https://3dwarehouse.sketchup.com/model/d0c8c865-c776-47bc-9a12-01f6466da15f/SFF-ITX-PC-Case-126L
Ie compare it with the desktop-laptops like Eurocom Sky X7C (i9-9900K, RTX 2080) which are close to 6kg and obviously incl. display and keyboard (which you would have to add to a sff-pc - ie either carry or ensure you'll have them where you're going to use it). But the extra weight (and inconvenience that you can use it on your lap) of such sff-pc can be offset by the better & quiet cooling allowing for overclocking, and much better upgrade-ability, compared to any laptop.
Just keep in mind that if you disregard mobility of such pc it become unimportant which case is smaller/lighter/better-optimized, as it'll be almost the same if it's a 7L case or 15L case.
