Sky's systems have probably made a mistake. At worst anyone with common sense can see you would never have deliberately removed 4 devices.
I had a strange problem last year. I removed a device to make way for a new device I was expecting to get soon and before I could add the new device Sky had filled the slot with another one of my already registered devices. I wasn't happy but they wouldn't budge.
So I cancelled (well, gave 4 weeks notice anyway). 2 weeks later and I was no longer cancelling, with a discount on my package and all the devices I wanted in my Sky Go slots, just saying.