UE4 is capable of doing some very pretty things, and over time its capabilities will improve, however for what you want to do a lot of it depends on your skill and how much time you have to do it. Realistically, it’s too much of a challenge for a single person to do, it would be pretty much impossible even for a team of professionals. Some really skilled developers could make a really large world, but it wouldn’t be high quality or have much to do, that’s always the tradeoff, time and money.
You also have to consider what the point of a project like that is anyway.