Streaming way too many levels

So I have a question I was thinking about this morning.

If I wanted to make a game where the whole game seamlessly streamed together, how would I do that? Like a whole interconnected world like from Zelda or a Metroidvania, using level streaming so you never had to have any transitions between areas.

I haven’t thoroughly studied how level streaming works in UE4, so maybe this is really easy and a moot point now, but I remember when I was working with UE3 for UT3 and UDK, you needed to load a persistent level that would load all the other levels. This was fine for the game because it would only load levels when it was told to, but for editing it would load every single level within the persistent level when you loaded the persistent level for editing. If I tried to have effectively the whole game load all at once, it would be too much for my system.
Another factor was certain things that couldn’t interact with corresponding actors in other levels. Like pathing, I recall, I had to keep all my paths in one level (even if there was no geometry around said path) because the path node could not connect to each other if they were in different levels.

So I’m curious how things have changed with UE4. If I wanted to make a single massive contiguous world within UE4, would these issues still be a problem? What other problems would I face?

Just curious.

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