To be honest, I have, because I know how tricky it can be to get proper multi-threading running.
I quickly checked.
https://wiki.unrealengine/Multi-Threading:_How_to_Create_Threads_in_UE4
He talks about multithreading in UE4. It is just a prime number test case, which can easily multithreaded. Multithreading an entire game is more difficult.
But it shows that UE4 is not multithreading well out of the box. Otherwise he would not have to set it up by himself.
It is just a small example, but any other thing that you create in your game will have the same issue of not being properly multi-threaded.