Looks like you have hit the one change per month limit, where you can remove as many devices as you like, but only add one per month.
If you contact the Help/Chat service and explain your circumstances, they can likely help you out here.
I would like to know what logic behind the four devices/one change per month is, though; Netflix let you have as many instances of Netflix available as you like, and to use whichever ones you like when you like, subject only to limits on how many sessions you can have going simultaneously. Which doesn’t seem to impact their revenues too badly 
The 4/1 limit was a nuisance when you had to wander round the house with the one working NowTV box and its trailing cables in your hand for the rest of the month, moving it from TV to TV, but now we have the stick, it’s just a very minor annoyance.
But as we can so relatively easily defeat NowTV’s intention, whatever it is, on TVs if not on tablets and laptops, why do they persist with it?
