It's not quite that simple.
If you have never used a device slot before, there is no limit on how many you can add. So for example, as a brand new customer, you would be able to add all 4 devices in the first month as you would never have used them.
Another example; you have Sky Go with two devices registered and upgrade to Extra you would have two new slots which you would be able to fill immediately.
Once you have used a slot, the one change per month rule applies.
To summarise:
Using an unused slot for the first time does NOT count as a change.
Removing a device and re-adding it DOES count as a change.
Adding a device to a previously used slot DOES count as a change.