Only if the CPU and motherboard support quad-channel memory, which is usually only found on High End Desktop platforms. Even the high end consumer CPUs you might consider for a new build - like a Ryzen 9 9950X3D or Intel Core Ultra 9 285K - are limited to dual channel.

In Ryzen’s case, the officially supported memory bandwidth is reduced when you populate 4 modules. For the aforementioned 9950X3D, you’re guaranteed support for 5600Mhz with two modules and that drops down to 3600Mhz with four. Whether or not you actually have issues will depend on the properties of the unique CPU you end up with / silicon lottery.

Depends how many memory channels the CPU supports. On a small consumer CPU, no benefit, but start getting up into bigger chips like Threadripper or especially Epyc and some of them support even more than four channels.

(Also, it might be worth noting that I'm pretty sure the Steam Machine only has two RAM slots, let alone channels. 4x8GB won't even be possible.)

Might not be noticeable enough, smaller memory usually got better timings but a lot also comes down to the CPU you'd ultimately end up using and even the mobo to some degree. That's assuming you're comparing same memory chiplets from same manufacturer packaged by same seller brand.
Unless you're really hammering the memory, as long as it ain't completely shit frequency/timing combination it wouldn't matter to most of us normal users.

Interesting. I haven't found it, but I did just spot the pinned comment in under the Gamers Nexus video: "TEAR-DOWN coming up next!" So I guess I'll just wait for that :)