If I can get an X-T4 out of Fujifilm at the same time (and I'm trying), there's a really interesting "three sizes of the same sensor comparison". I also have an A7r IV. All three use the same sensor generation, the same pixel pitch and, in fact, the same pixel structure. They're really the same sensor (or as close as you can get) in three different sizes. They unfortunately use three different sets of color filtration,and the X-T4 has a higher rated base ISO, although there's quite a bit of talk that it may be oddly rated, and closer than one might think to ISO 100 on a Sony. I don't know if the GFX is more similar to its Fujifilm stablemate or to the Sony (which is generally thought to be pretty accurate in terms of ISO.
It's actually rare to come up with such similar sensors in a range of sizes - the 24 MP APS-C sensors aren't related to any full-frame or medium format sensors, and the 24 MP FF sensors are much lower pixel density than just about anything else. They aren't actually related to the 50 MP medium format sensor (they're slightly less dense, and a different design) The 42 MP Sony and 46 MP Nikon FF sensors seem at first glance like they might be related to 24 MP APS-C sensors, but they don't turn out to be. This trio is, in fact, the closest set of relatives in a long time (if not ever)...
There is one possible addition to the comparison that I won't be able to come up with at the same time - the $50,000 Phase One IQ4 150. It's the largest version of that same basic sensor I don't know if I might be able to come up with one somehow for a few days at one point or another, but almost certainly not while the two Fujis are here.
The Sony, of course, has different processing (minimized, but not eliminated by shooting Raw), and the X-T4 is X-Trans (again, a variable that is hard to eliminate). To add further complication, the GFX is a slightly different aspect ratio...
24x36" prints of the same or similar scene from the three cameras on the same printer would be very interesting. I'll try and do it, shooting landscape in Yellowstone NP. It would be the closest anyone's ever come to measuring a pure resolution effect - even with all the variables, every other combination I can think of is worse.
Replacing the X-T4 with a Pentax K3 mk III (I don't think I can come up with one at the same time) would eliminate the "one camera is XTrans" variable, but it would add another variable - a third set of processing and algorithms - plus I'm not sure what lens to use on the Pentax.
The two Fujifilm bodies share quite similar default processing, and should behave similarly in terms of any noise reduction applied to raw files (etc.). The Sony will, of course, differ by more... XTrans throws in a demosaicing challenge - I can't use DxO (my preferred high-end converter) for all three, because DxO doesn't like XTrans. C1 for all three? Adobe would work, but would be a lowest common denominator...
Of course, there's no lens that will go on all three bodies easily. There's really not even a lens that will go on any two. The closest thing would be some manual lens that was adapted on all three bodies (the ideal comparison would be the 85mm Otus on everything, because that would be darn close to a perfect lens on each camera...). I don't still have the Otus around, nor do I have the right collection of adapters... Of course, even that would be opening up field of view as a variable - I think it would be most interesting with matched fields of view (and that's going to mean three different lenses, and probably all zooms).
Even with three different lenses and trying to get the post-processing close, but knowing it won't be exact, it should still be revealing...
24x36" prints on the Canon Pro-2000 should be pretty decent comparison fodder, no?
Let me know about any other considerations anyone thinks should be included...
