I also got to play around with the new Pro 4... My store sadly doesn't have the Surface Book Yet, and is currently petitioning for it.
I Love the new Keyboard, Pen, and Larger, Higher Res Display (same foot print but larger display due to smaller bezels). The MS rep told me since I have a Pro 3 It would be better just to buy the new pen and keyboard, instead of getting the new Surface but from the iris scanner, to the thinner body, to the larger and sharper display with slimmer bezels, and improved cameras all the way down to the new shiny MS square logo... I STILL WANT IT
I would set the Surface Book as my goal, but since I'm more of an "Artist/Creative Software" based Surface™ User who is often dabbling in Toon Boom Harmony, Sony Video Editing Programs, and Art Rage instead of tasks that are more suitable for a laptop, I don't think I would really benefit that much from it... I like the idea of the larger display, but I like how the Pro series has the flippable keyboard that can simply be folded back, and you can ease the Surface lower with the Kickstand to create a Cintiq-like set up... With the Surface Book, it appears to be Laptop, Clipboard, or clipboard with a keyboard on the back.... from what I've seen in videos the hinge doesn't look stiff enough to hold still while someone rests their palm on the display and pushes down their pen all over and across the display. The screen would simply have to lay on the way down, which creates a slant that slopes down towards the direction I am facing which is the opposite effect artist want IMO.
Surface Pro 4- Tablet/Small Clipboard, Laptop with light thin board, or angled display with keyboard out of the way
Surface Book- Large Tablet/Big Clipboard or Laptop with Sturdy board
(taking the tablet off, flipping it around and clipping it back on backwards essentially achieves the same effect as using it as a stand alone tablet, just without USB ports, and the extra powerful GPU ( onto the S-Book only) which is why I don't include the positions of having the keyboard folded on the back of these devices)
The maxed out S-Book essentially has the better gpu, more ports, thinner chassis, larger display (same PPI), and sturdier laptop mode going by videos, review and spec sheets.
I would somewhat like the Surface Book more for the better gpu, extra USB port, thinner chassis, LARGER DISPLAY, and sturdier laptop mode.... but I like how quickly I can switch between laptop and "Cintiq mode" on the SP3... but to really come to a firm conclusion I'll need to actually experience for myself this 'ultimate laptop' going by the name of Surface Book... It might just in fact be able to perfectly produce the 'Cintiq mode' position just fine and I'm just a stupid mouse.... I'll find out