Right, so you’re going to want to use World Machine, make the tiles and use World Composition inside of UE4. If done correctly, there shouldn’t be any issues with the edges lining up and you should be good to go with level streaming. Just gonna take a lot of artists to work on maps of that size.
Edit: Just reread the OP and realized that you’ve done this process before, so not sure what you’re confused about. The workflow is sound, but ioFlow is correct about multiplayer origin shifting issues. There was a pull request for a multiplayer version of this that was merged in 4.14 but apparently it’s limited (at least that’s what it read like) that the feature was limited to 20km by 20km worlds, which is the max world size without using World Composition if I remember correctly. In that case, you may have to talk to your client about finding a custom solution as Epic used to advise users to do.