If you subscribe to the DVR service you won’t have to purchase any apps. Just install the Channels-DVR version of each app.
If you aren’t subscribed to the DVR service then you’ll have to purchase Channels Live apps once for each platform per account. E.g.: Here in our household we have two Android TV boxes, two Amazon Fire TV boxes, and my wife and I each have an iPhone and an iPad. So we had to purchase four licenses: One for each of the two STB brands and one each for each of our two iThings accounts.
In your case: If all four of your TVs have the same brand streaming device, and they’re each associated with the same account, you’ll have to purchase a live app only once. And, again, even then only if you don’t subscribe to the DVR service.
Hope this helps.