If it adds a real 8 hours of life with MS Office, I would buy it. I would expect it would be about the same size and weight as the SP4 itself, and would cost around $200-250. Microsoft gave up too easily on its old one, which is undersized for the application. The Surface Book might evolve into the right solution if they put a bigger battery in the tablet and get rid of the "gap" between tablet and keyboard. And of course iron out the bugs.
Or they could accomplish the same goal by making the battery replaceable.