What’s the difference between NEK 6 & NEK 4? What did they add (or subtract)? Could I still use NEK 4 with Nuendo 6?
Don’t know what has changed, but the answer to your second question is easy; no.
DG
Is that because of some kind of driver compatability issue? Would it work with NEK 5? I’m not clear why I need to keep spending the extra $150. So, if I upgrade to N6 without NEK 6, then I’ll have to do all of my MIDI prep in N4 and then import it into N6? 
It’s very simple. You can only use the NEK for the version you have paid for. Therefore if you don’t pay for N6 NEK, you don’t have a licence to use it, and the N6 NEK features won’t appear in the N6 menus.
DG
Unless theres some kind of financial motivation (like, its a LOT cheaper to by it with N6 instead of separately), I’m going to have to wait until I find out what other benefits NEK 6 will offer then. I’ll just consider the N6 solo upgrade.
I would also consider upgrading to N5, as for me the upgrade to N5 was cheaper that the upgrade to N6.
DG
That’s the plan right now. But I would like to know what they did to NEK 6. So, if anybody finds out, would you please post that info? Thanks!