Wysardry

I'm not sure of the details of how UE4 handles objects in different map chunks, but I do know that there is a default or master level that always stays in memory regardless of the load state of any others. Maybe they store object data there?

Yes, level streaming could also benefit those creating maps within the current size limits. Splitting a 4k by 4k map into 16 chunks of 1k would reduce the memory and processing requirements whilst playing the game, plus it would allow several people to work on different parts of the map at the same time during development. Map streaming would also allow for sizes and shapes of map that aren't currently available. For example, using 1k "chunks" you could have a 3k by 3k map, or even create non-square maps, such as 4k by 2k. It would also make it easier to create expansion packs that don't seem "tagged on".

It really depends on the type of game you are making whether the current map size limit would be big enough or if you would need to fill every square metre with hand placed content. If the player had access to fast transport - such as a car racing or flight sim game - they may be able to cross even an 8km wide map quite quickly, yet they might not expect areas away from the main road or flight path to be filled with interesting stuff. For these types of games, loading screens would be a distraction. It's also possible that the purpose of the game is for the player to place interesting content in undeveloped areas, such as with games like Transport Tycoon (with a more realistic scale). In that type of game, this engine's procedural tree and rock placement would drastically reduce the development time and memory footprint. There are quite a few third party tools for creating huge terrains (some planet sized) and/or generating procedural content, so there must be an interest in making larger games. The main reason we don't hear of many being attempted is that so few lower priced game engines can handle them.

As large maps can suffer from floating point accuracy problems, are there any plans to add some sort of streaming system to terrain maps, where the world origin is reset to zero when the player enters a new section/zone, as in Unreal Engine 4? CryEngine was also going to have a similar feature added (called segmented worlds), but I'm not sure if that ever happened for public releases. Populating such large areas should be much easier - even for smaller teams - with Leadwerk's procedural vegetation system.