What are the pros and cons of the Microsoft Surface Pro 4 for video editing?
As you mentioned, mobility would be your biggest pro by far. With the Surface Pro with highest specs (as you mentioned), you'll get a very competitive machine. The biggest thing you'll be missing (from my perspective) is a dedicated graphics card. You can probably still pull off multiple monitors, but it's not going to be pushing it for sure. The Surface Book actually does have a dedicated GPU, and could combine that mobility with a powerful graphics card (it does come at quite a bit extra cost, though).

The best thing I've found with my Surface (a Surface Pro 3, admittedly, but still in the same family) is that I can connect a mouse and external keyboard via Bluetooth (was never a fan of the Type Cover) and still have my USB port free.