The newer keyboard does indeed fix many things, starting with the track pad and the key spacing. For function key users, the Fn key is moved to the right place, and is also a lock key, also a huge improvement for those who care. The Pro 4 also fixes many issues on the Pro 3, but it's more of an evolutionary refinement.Maybe Windows 10 makes all the difference, or the apparently-better newer keyboard case makes it more pleasant, or the Surface Pro 4 fixes everything--I’m curious, because I wanted to recommend one to my artist but that was really difficult after actually using it.
It's hard for me to compare Win 8 with Win 10. I liked 8 a lot, but also suffered through the many insider builds of Win 10 on my Pro 3. Win 10 has evolved into a very good touch-based OS, but it's not much like Win 8 any more.
The real gem of the Surface product line has to be the Surface Book. The laptop with detachable screen works better for me than a tablet with a detachable keyboard. The trackpad is every bit as good as any Apple trackpad (and yes, I also own a MacBook Pro and love the trackpad on that as well). The new Surface displays beat anything from Apple that I've been able to compare, specifically the iPad Air 2; brighter, better color, and equal pixel density. I have not been able to compare with the iPad Pro yet. My "pro" use cases are Visual Studio and Photoshop, so the iPad Pro really isn't something of interest for me.