Re: Adding a roku device to my plan to watch a max of 4 at a time
What do you mean by "activated under my plan"? If they are all linked to your Roku account at https://my.roku.com they can all be used simultaneously. Note that some channel subscriptions may limit how many simultaneous streams you can receive (e.g my Netflix account is good for two, my Acorn says it's limited to one but I have been able to use two).
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T