Can't you have 1 box wired and 3 mini boxes?

You can only have 1 Sky Q main box (wired to the dish) per property. The other rooms, you then get Sky Q mini boxes, which connect to the main box wirelessly.

What are the limitations on the mini boxes?

What's the deal with Netflix! Would it only be available on main box or also the mini boxes?

We have one Sky Q box in the loft, and the we distribute 3 hdmi channels over Cat6 to the TVs IR blasters in the rooms to control it, works seemlessly, of course if you want to watch different things in different rooms you're either using your TVs antenna or a firestick or whatever, but we're not paying for loads of boxes. And we're always in the same room so it isn't an issue. But it's an option.

We often have all 4 on the go, different viewing habits!

You need to haggle them down on the mini boxes then!

No 4K, and no storage, all recordings are on the main box.

Netflix and the other apps are on the minis too, and are independent of the main box, and the number that can be used concurrently depends on the level of your Netflix subscription i.e. number of screens.