I upgraded from an SP3 (didn't trade it; still have it). I am much happier except for the following annoyances:
1) I have three docks and the SP4 doesn't fit just right. I am trying to get the promised spacers to fix that, but no luck to date.
2) Video driver crashes while web browsing. Comes back, however, with minor issues.
My SP3 would run hot, not come out of sleep, crash, etc. I went through five copies of hardware and never could this be solved. So far, SP4 is rock solid with none of those issues. I also like the (slightly) better screen and the new type cover is much better! (Granted, it would work on the SP3, also, so upgrade that if you'd like a better typing experience.)