Questions of a project with large 3d scence

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Hi my dear friend,
My company will set out to start a new project ,stimulating a city with the size about 800k㎡ in computer. I have the intention to use the engine of UE4 .But there’re still some questions puzzling me.
Firstly,Can ue4 run the scence of that size smoothly? That is to say what size of 3d model, referring to the quantity of objects and faces in the 3d model,can run smoothly in the UE4.
Secondly, is there any optimize measure to deal with the large 3d scence by program way that I can get support from you additionally?
Thirdly,this project will last a long period of time without dout,probably one or two years.Can the lastest version of Ue4 in the future still be compatible with the project i made by the old version .
I will appreciate your kindness if all these questions can be explained clearly.

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