Modified version of Unreal Engine 4.
When they say "... a modified version of Unreal Engine 4 was used":
For example, Days Gone: "... the game uses Unreal Engine 4- a modified variant of the game ..."
asked Dec 10 '17 at 10:31 PM in Using UE4
Check out this link (https://www.unrealengine.com/en-US/ue4-on-github) and if all goes as it should, you can have the UE4 source code in a matter of minutes. If you ever want to do client/server development, you will need to use the engine source anyway, so it's good to have access. You won't need to make changes per se, it's just that building a server requires the source code.
Just be warned that you are opening Pandora's box. ;)
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