There will be some initial map built by us. Player are able to modify this map, eg. dig out tunnels, place down items and similar actions. Whole game will be backed by dedicated server running for a very long time.
I have found this wiki about about save system, but it’s most probably not multiplayer compatible. I mean server would be probably able to read from it and use it, but problem comes with clients. It would need to replicate to them immediate vicinity of the level and then gradually send move pieces as player moves through the map. Is something like that even possible?
If that would be possible, then perhaps the best approach would be to actually build the level with the in-game tools we would have to create and always stream data from authoritative server instead of packaging client with map data.
Follow up forum thread: Persistent world with UE4 (not an MMO) - General Discussion - Unreal Engine Forums