Anandtech reviews the SP4 and finds:
* The heatsink spreads the CPU temperature around the entire laptop.
* Much less CPU throttling because of the better cooling
* The core m3 version is indeed fanless
* Type cover is significantly improved, stiffer with better key travel
* Pen and dock are noticably improved
* As a tablet it is the most powerful one yet
* As a notebook the i5 SP4 is about as powerful as Thinkpad X1 (i7-5600U)or Acer S7 (i7-5500U) and is about 20% more powerful than the SP3
* Graphics performance is much better than SP3, and better than the two ultrabooks
* SSD performance is much improved
* Display reaches 377 nits max, more than SP3 but less than S3
* Blacks are deeper than on SP3
* Battery life in browsing and light use is the same as SP3
* Battery life for video playback and heavy use is significantly better than SP3, but less than the ultrabooks
* Wifi card is identical to SP3