There was a tutorial series on 3DBuzz a few years ago about making an MMO in Unity. They made a level tool that packaged up the terrain into grids. You could make a separate UE4 project to make the level in, sculpt out the landscape etc. but then cut it into a grid and save it into a folder structure of tiles, with markers etc. for all the objects. Your game could then have a loader that grabbed out the tiles from their folders. Just an idea.