If you want to load a completely independant level (outside of your current persistent map that is), you would use the common Open Level node provided by UE4 (this will unload the currently loaded world). To still be able to have a loading screen during that transition, that destination level would have to be set up for level streaming as well, i.e. have a persistent level “shell” of its own (with just its LSS actor) and the actual level plus a loading screen scene as sublevels.
The two (persistent) levels won’t share the same LSS actor nor the loading screen scene, but the observable effect will be the same.
This type of transition is showcased with example map no 11 (“\Content\LSS_LoadingScreenSystem\Maps\11_OpenLevel_Map”), including a fade out during that transition (before the Open Level node is called).
In other words, the second level would have to be setup with a “startup” loading screen, as seen in this video:
