I emailed a couple Microsoft guys (Peter Bright at Arstechnica and the guy at AnandTech). They pretty much said the same thing.
The new pen will perform better than the old pen in terms of latency after a firmware update (no timeframe for when that firmware will be released) on the Surface Pro 4. But even after an update, the latency of the pen will still not be as low as the latency of the new pen when paired with the new Surface Pro. So if you demo the new pen on the new Surface Pro at the store and then buy it and go home and try it on your SP4, you might be a little disappointed.
Tilt will also supposedly come with the firmware update.
But they have no clue about whether the increased pressure sensitivity will ever reach the SP4.