Versions and use
Avalanche Engine 1.0
Used in Just Cause.
Avalanche Engine 2.0
See also File types in Avalanche Engine 2.0.
Avalanche Engine v?
Their current open-world engine, used in JC4. Avalanche Studios seemed to be pretty proud of it. In trailers, interviews and promotional content, this engine was constantly mentioned. It is said to be an advanced game engine that greatly increases the game's capabilities, including (but not limited to) better graphics, improved terrain sculpting, more sophisticated physics systems and most of all the addition of weather-related features that allowed the developers to program extreme weather.
Unreal Engine 4
Used in Just Cause: Mobile.
Official site for the engine: Link.
JC3 developers talking about the Avalanche Engine and how it's been improved since JC2:
JC4 developers talking about the Apex engine: