I stopped by a Microsoft Store yesterday and checked out the SP4. They had one sitting next to an SP3, and I was struck by how much colors popped on the SP4 compared to the SP3. Contrast was also significantly improved, and color temp was much more pleasant (the SP3 looks way too cool by comparison). Microsoft has done something special with that screen--their work on colors and contrast is showing some real dividends.
It's not enough to compel me to upgrade, at least not in my current financial situation, but I admit to being unexpectedly tempted. I love my SP3, and it performs perfectly well for my needs, but man, the screen on the SP4 is something special.