The rules are that you can only add one device per calendar month, so the "count" is reset on the 1st of the month.
You can remove as many devices as you want over the month, it is only adding a device that counts as a change, even if it is a device you have used previously and removed.
So, if you have already added a new device this month (i.e. since 1st September) you will need to wait until 1st October to add your PS4.