I had the opportunity to try out the Surface Book i7 with dGPU 16GB ram model. The machine looked and felt great, the screen size and resolution is amazing. As far as the GPU is concerned, I would say the closest comparison I can make is to my MacBook Pro 15" 2014 ( uses the GT 750m ) I was seeing VERY similar speeds from the default project templates. If your plan is to use desktop style graphics features, then you will be VERY disappointed. If you are doing low poly / 2D, basically making a mobile targetted game, then you may be VERY happy indeed. The portability, the ability to draw, the disconnectable screen ( though you won’t be using the dGPU at the moment ) is amazing. Please be warned, that Surface Book as described is $2700 ( funny enough that was the price of my MacBook Pro 15" ). If you have the cash for it and your project fits what you can get performance wise, then this may be an awesome idea for you.
As far as the Surface Pro 4, I also had a chance to try out the i7 model with the Iris GFX. The performance was a bit more then half the performance of the Surface Book i7 with dGPU. I would say that if you are doing purely sprite 2d games in UE4, you probably will work out fine.
Hope this helps!