The Pro 4 has a little larger screen than the Pro 3, with much higher resolution (2736 x 1824 Vs. 2160 x 1440) and they've improved the touchscreen technology – the new screen measures the amount of light blocked by whatever is touching the screen to determine whether it's a finger or a stylus and then touch detection is optimized for whichever it detects. The new Surface Pen is more sensitive (1024 levels of pressure sensitivity) and has an "eraser" on the back end – it comes with the Pro 4 but is available separately for other Surface devices (for $60.)
You can get up to 16GB of RAM and 1 TB SSD and a new Intel Skylake processor – they say it it is 50% faster than a MacBook Air (no mention of the actual models compared…)
It has all the same wireless options as the 3 (802.11ac, Bluetooth 4, USB 3, mini displayport, microSD reader.) They still use the "SurfaceConnect" charging port (instead of the new USB-C that many others have moved to.) You still can't get a 4G radio in the new Surface. The rear facing camera has been upgraded to 8 MP (from 5 on the Pro 3). Battery life is the same ("up to" 9 hours.)
It's a little lighter (1.7 lb vs 1.75 lb) and about a mm thinner.
There's a new "Type" cover available. It has a larger trackpad, a fingerprint sensor and wider key spacing. It's still extra cost ($160) and it works with the Pro 3.
The base model starts at $899 and fully configures goes up to $2699. It's available for pre-order starting today and will go on sale October 26.
They make solid hardware so if you're looking for a beefy laptop, this might be a good (albeit expensive) choice.