In case of the normal mobile processors, a fan-cooling solution is mandatory. However, if the rumors are true, Microsoft seems to have developed a fan-less design with small vents that allow hot air to flow out of the chassis.
Other speculations suggest a 12inch display with a resolution similar to the Microsoft Surface Pro 3 (2160×1440). Apparently the Surface Pro 4 could be thinner than its predecessor but quite comparable both size and weight-wise. According to Design & Trend, the Surface Pro 4 could feature a bigger 14inch screen with the same 2160p display resolution.
In the meantime, Microsoft has been reducing the price of the Surface Pro 3 for quite a few months now and it seems like the company wishes to a maximize the profits. Recently, Microsoft offered a $150 price cut to the various i7 and i5 variants of the Microsoft Surface Pro 3. The i3 model did not receive a discount and the entry-level Surface Pro 3 model is currently priced at $799.