for MS4 which iMac?

I have the M1 MacBook Pro from 2020 (8 core CPU, 8 core GPU, but that one extra GPU core is really not going to make any difference), and MS4 runs perfectly on it (in fact, I even use that MacBook for developing MS4!). I think MS4 runs fine on any somewhat modern Mac, especially Apple Silicon ones.

Basically, the heaviest thing you can do in MuseScore is Muse Sounds. For very large orchestral scores, you might notice small glitches on low-end machines. If I do heavy tasks on my MacBook while playing back a heavy score with Muse Sounds, even on the M1 chip I occasionally hear a small glitch. But really, it's not disruptive in any way. And if you don't need Muse Sounds, but simply use the built-in MS Basic sounds, then everything runs very smoothly.

So my advice is: in terms of CPU power, you can safely go for the "cheapest" M1 iMac.

Another thing to consider is storage though. Muse Sounds takes 15GB, which is quite significant if you choose the 256 GB SSD. So you could consider choosing a larger SSD, or you could work with external SSDs, which is often much cheaper. (At this moment, there are some bugs with installing Muse Sounds on external disks, but that's being worked on.)

It's a very real concern for those of us with older Apple non-mobile hardware, WHICH version of macOS is used. If you're buying new, this likely won't be a concern. And since you know now that MS4 + MuseHub / MuseSounds can utilize significant disk storage, your choices in that area should tend to be "more free space is advisable".