From Riftui Wiki
Addons written for older versions of Rift must be intentionally allowed to run by choosing to run out-of-date Addons in the Rift Addon window.
Environment = "4.0"
- 4.0 - This is the current environment. It came into effect with the Starfall Prophecy expansion.
- 2.5 - This is the environment that was in effect when the beta Addon API was released to the Live Shards.
- 1.3 - This is the environment number when the alpha Addon API was released on the Public Test Shard.