This is what is happening.
Windows 10 is indeed an iteration of Windows 8 - but this does not mean it was “completely rebuilt”, like your original claim. UE4 when it first became available had a new renderer, but otherwise was very similar to UE3 in most aspects. Over time, many of the tools and features have been replaced by newer ones, making it what it is today. Slate UI, Blueprints, Landscapes etc. all have their roots in UE3.