If you have no game development experience then I would just abandon this kind of idea, because the most similar types of games are very difficult to make.
On a more practical matter, it’s not something UE4 was designed to support. It has the ability to combine maps to make larger worlds, but that function does not work in multiplayer. It’s possible to create a solution that works with multiplayer but obviously you have to actually make a solution for that.
Besides that, you get other issues. If you design the whole “world” then you run into an issue of storage space, it will be impossible to work with and too much data to install. Most games that want to do world-scale type games (like No Man’s Sky) will use some procedural method, where the world doesn’t actually exist until you get to it. The problem there is trying to make the generated results look good, because it’s not going to look as good as it would if you place everything yourself, in terms of sculpting the landscape and placing things like vegetation. Also, you would have to come up with a way to make that function as well.
The biggest thing though–Why would you want a game like that? What does an entire planet offer over something like a very large map? People keep mistaking size for substance, you aren’t making a game better by making it as big as you can, you’re just spreading your resources over a larger space.