The free version of Sketchable only installs one brush and an eraser, but it was more than enough to provide a better sense of the pen experience on both generations of Surface Pro tablets.
Although the new Surface Pen won't add additional pressure levels or reduce the initial activation force on the Surface Pro 3, it does make a world of difference and I can absolutely recommend that all SP3 owners consider upgrading their pens.
The resistance offered by the new nib is just right (your mileage may vary!). The pen eraser feels exactly like a rubber eraser. As you press down harder on the screen, you receive tactile feedback as your stroke thickens.
To make sure I wasn't just imagining things, I went back and tried the original pen on the Surface Pro 3 and the difference was very dramatic. No feedback whatsoever as I increased pressure. Drawing on the SP3 had never felt so glass-like.
Turning to the Surface Pro 4, the lower initial activation force was immediately evident. Even the lightest strokes are recognized (and this is without any pressure curve tweaking in the Surface app). The increased pressure range is apparent as strokes move from light to heaviest.
The hover distance appears to have increased and is certainly larger than on Wacom Active ES devices I've tested. The hardware cursor still lags as you hover the pen, but it is absolutely aligned with the pen tip as it touches the display.
According to Microsoft, parallax has been improved on the Surface Pro 4. I haven't been bothered by parallax on current gen Surface Pros, and I didn't notice any issues here.
The brief video below illustrates the performance I saw. Apologies for the shaky video. It's hard to shoot and draw at the same time.