I'm hoping I can come up with a Phase for a few days in late May/early June...The rest are arranged (or, in the case of the Sony, I own it) Does anybody know if Capture Integration still has an office in NH? they used to have a branch in Manchester (which is conveniently located between Boston, where I'll be for about three weeks, and the White Mountains, which are great landscape photo locations). Better yet, is Steve Hendrix still on the MF forum?
I can't speak to Steve/CI. But DT has a physical office in NYC (easy train trip; our office is a 5 min walk from where the Boston train gets in) and a full-time employee that lives full time in lower Maine 2-3 hours from the White Mountains.
One of the big questions is "just how big do you need to go to see the differences"? I'm sure the Phase would win a 10 foot print viewed from two feet away, but that's undisplayable in almost any circumstance.
My experience is that people come for the resolution and stay for the dynamic range, color, rendering, and work process. Having more pixels is definitely helpful, especially for cropping and big prints, but even on a small print the dynamic range, color, tone, and rendering of a Phase One file is hard to beat. Re the work process having tools like in-camera automatic raw-file frame averaging, the in-camera behind-the-lens movements, the physicality of the system etc all create a very different capture experience – notably some people will love it and some people will hate it; it's much closer in mentality and process to a 4x5 than an iPhone, and that isn't universally a better or worse thing.
Where does the X-T4 fall away from the pack? I know it eventually does, because I have good APS-C and good high-res FF prints on a wall, and the difference is visible at 16x24" and glaring at 24x36". The APS prints are X-T2, so the X-T4 is better, but it's not THAT much better...
I own a Fuji XH-1, an iPhone 12 Pro Max, and can use any of our P1 gear at any time. In my opinion you're asking the wrong question. It's really not about print size; it's about what and how you are shooting. When I have time, space (physical and mental), and opportunity (e.g. the subject matter is conducive to being shot on P1), I will always use the Phase One; it's rare that my motivation is the increased resolution, though it never hurts.
I'm not a tech camera user (yet), and I wonder if learning one in the limited time I have would be realistic...
I've been on plenty of workshops with photographers who started the week with their first-ever tech camera experience and were perfectly comfortable using it by the middle of the week. There is a learning curve, and on a short trip there is definitely a trade-off – it's always possible the best sunrise of your life will happen when you're still learning the ropes. Generally what I'd recommend is to set up for any given scene and shoot first with the camera you're totally comfortable with, and work the scene until you're comfortable you have something you're proud of. Then switch to the new camera and expend any remaining time reshooting the scene with the new camera. That poses the least trade-offs in my experience. It also gives you reasonably similar images to compare to see if you think the results are worth the various costs.
As for any comparisons, I encourage you to look beyond direct head-to-head image quality tests. Those are of some value (e.g. pixel peeping detail and directly comparing color) but they address a fairly narrow range of questions, and spoiler alert, the one built at a higher cost for a narrower range of goals (the P1) will win. Such tests also tend to converge toward the lowest common denominator in order to keep everything equal-and-fair. Do those kinds of comparisons; get them out of the way. But after that, and once you're familiar enough with the P1 that you're thinking more about the image than the machine, I encourage you to spend at least a day with each system as the only one you're carrying, and work each scene based exclusively on what that system allows. That is, use each to its absolute fullest the way you would if that was the only camera you had to explore the scene and create images. That way, your comparison is more holistic and, in my opinion, realistic. Reality, especially the reality of art, is messier than theoretical lab-based all-variables-controlled. If you connect with a camera and its work process, and you enjoy making images with it, and its modality of working fits your modality of creating, then you will get better images from it. That goes either direction; if the Fuji (or iPhone, or Sony, or film, etc) suits you better than an XT IQ4 then any resolution/detail/lens/color/DR comparison is moot. I can promise you that you will not be disappointed with the image quality from P1; I can't promise you that its work process will suit you – only you can determine that. Likewise I can tell you that you won't be disappointed by the acceleration or cornering of a super-car, but I can't tell you if you'll enjoy driving it. So, in any case, it's good you're thinking about testing.