ExoPlanet IV: Sunset is a standalone addon (Skysphere/HDRI) for ExoPlanet IV.
You can use it with any environment you like, the main set is not required.
Aside from the change from daytime to sunset, the moons on the horizon are much smaller than on the original skysphere.
A bit more realism, a bit less dramatic effect. ;)
The file formats are native .DUF for Daz Studio 3Delight and Iray, and .PP2 for Poser.
Lighting is included for all versions (Iray through the HDR environment, 3Delight parented to the skysphere, Poser as separate preload).
You can get additional info and tips here: http://flipmode3d.com/easy-environments/
-Note for 3Delight / Poser users:
If you use this addon with a scene which already has a skysphere loaded (e.g. the ExoPlanet IV scene), you should delete the original skysphere from the scene to avoid overlapping.
-Note for Daz Studio / Smart Content users:
The ExoPlanets sold here at Renderosity do include Smart Content (Metadata), but I think Daz Studio doesn`t automatically read metadata for products not sold at their store.
If you want to use it you can go to the little options menu on top of the Smart Content tab, choose "Content DB Maintenance" and "Re-import Metadata".
You should find the ExoPlanet metadata in the product list that pops up.