The AR is quite possible a better piece of kit on it’s own than any of the other machines up for discussion here. I love doing chromatic stuff with single cycle wave forms, it’s way easier than the Octatrack. I don’t want to seem too down on it, because it’s really a fantastic machine. (enough so that I’ve owned and sold it 3 times.)
On the other hand with the A4 I know I can go to it and recreate pretty much any kind of synth lines I need for any given song. It’s not the most exciting machine, but it’s utility is undeniable and it’s pretty predictable. PLUS if you really exploit sound locks / multi-map mode it makes a pretty good drum machine as well. It’s arguably as good if not perhaps better than the AR if you leave samples out of it and if you’re willing to either load up on sound packs or spend some time synthesizing drum sounds. (you won’t have a choice on the Tempest!)
For me the Tempest more than fills the hole left by ridding myself of the AR, and then some. It wades into A4 territory but there are just some things you want a synth to do that it doesn’t. (i.e legato / note slides / quasi-FM, etc.) But in terms of sheer sound it’s way more versatile than the A4 even with just one filter. It’s got tons of cool digital waves and a much better sounding analog oscillator in my opinion. I find the A4 oscillator to be pretty tame by contrast.
If I had only one machine it’d be a tough call between the Rytm and the Tempest. The Rytm is more self contained (sample playback, FX, etc.) but I always find the analog drum sounds to be a bit boring. And the Tempest has 1000% better pads.
With 2 machines, I’d probably go for the A4 and Octatrack, or perhaps a Monomachine and Octatrack. And buy lots of Goldbaby samples. I just gotta have a synth that can do portamento/legato/slides.
With 3 machines I currently use and am very happy with the Tempest + A4 + OT combo. The Rytm is definitely more exciting than the A4 but the A4 can cover some ground none of the other machines can. (mainly smooth and silky stuff.) It’s my most frequently used but least exciting machine.