I recently noticed that unreal engine now allows map sizes of up to 20km x 20km / 400km2 since update 4.6. Firstly can i check that this means i can now create multiplayer maps up to 400km2 and secondly how would I be able to achieve this without causing huge problems with performance?
This is achievable using level streaming, provided by World Composition.
It takes care about performance side of having and rendering huge worlds.
The multiplayer part of your question, and how to optimize it, is entirely up to you.