We have put together a short survey to capture your feedback in terms of what BT can do when you experience home Wi-Fi issues. We would appreciate it if you could take a couple of minutes to complete a short survey. Read more from here, Fix my home Wi-Fi
Thanks for the pointer, John. When I went back to adding the 4th disc, it connected straight away without the need for the factory reset. I guess it must have done an update in the interim. All working well now, devices switching seemlessly between the discs and full coverage around the house for the first time.