I've played 4 extensively but not VI so can't give you a direct comparison. But fwiw...
I used IV for build testing using it's 100% XP option and have therefore never truly finished it. I do plan to play it properly at some stage - so many modules, so little time etc.
I can confirm that 4 does have randomly generated items that can be quite powerful yes. But not always, it's luck of the draw. I'm pretty sure there's some smarts built into the scripting so the items are appropriate for your class. You also earn quest points during the game and can spend these on powerful magic items or adding enchantments to your gear too.
I found encounters can get very tough at higher levels which often means you need the gear. The henchmen mod (whatever it's called) where you can create your own party can help add a little more omph and fun.
If you ever played roguelike games and like them for the challenge and randomness but in the instance of NWN are ok going through the same areas during each play through then it's quite fun. There are a lot of area's so it's not that tedious. I think if you approach it from the standpoint of a survival challenge rather than a roleplaying/story mod you won't be disappointed.
Other thing is it's designed to be played from 1st through epic levels, highest I've gone is around the mid 20's.
Anyway have fun
-kwm
=)
