Surface Pro 3 was a huge success for Microsoft, that they going to cut down the price to compete with apple's macbook air. The next Surface Pro 4 would using the Intel Core M (like Lenovo ThinkPad Helix and Lenovo Yoga 3 Pro), with 13 to 14 inch screen (mostly would be the same screen ratio like Surface Pro 3, which is 3:2), and packed with Windows 10. Also, Microsoft would release only 2 versions of Surface Pro 4 (rather 5 versions like Surface Pro 3). Some said that it would have the same spec of the next apple's macbook air
source:
Chinese Version
English Version
source:
Chinese Version
English Version