Infuse pro 5 Payment

What is the difference between infuse pro 5 pay 1 time and the version for pay monthly?

The one off purchase allows you to use the Infuse 5 Pro feature for as long as you have the app installed and covers all future updates to Infuse 5 only (I.e. 5.1, 5.2, etc).

The subscription purchase allows you to use the Pro features while you have a valid subscription and will be valid in future updates including major revisions (Infuse 6, Infuse 7, etc).

There is also a difference on how to share the Pro feature with other family members if that is important.

IS this membership good for only apple tv 4? What happens when atv 5 comes out?

the subscription is good for any TVOS or iOS devices linked to the same Apple ID. Amongst other things t hat means if you buy the current version for the ATV4 it will also work on any iOS devices (iPhones/iPads) that you also own.