Dynamic sky materials are partly procedural. The easiest method is to have a few masks for stars, sun, moon etc. Atmosphere gradient can be done with masking out a channel of the UV texture coordinate, that would give you "from top to bottom gradient" that you can color with a lerp. The sky hemisphere mesh should have 2…
The default UE4 sky is a procedural blueprint so you could use that as a starting point? If you don't want to figure it all out by yourself you could look into this for example [ame] https://www.youtube.com/watch?v=lwi4K4x0Q3k[/ame]