Well, I'm not a Cubase user, but if I were, I would probably say that it'd be worth it just because of the upgrade to 64-bit, not to mention the new stuff like pitch correction.
Kontakt 3 probably wasn't a huge upgrade until 3.5 came out, which introduced 64-bit as well as a HUGE performance boost in streaming so that loading times are faster and waaaay less RAM is used. However, Kontakt 4 is out now, and it's not only got a better core library, but also the new attributes-based browser which looks AWESOME. Zzounds should be shipping mine out today and I can't wait to use it.
I'd say if you can afford both, the upgrades should be worth it.
Remember to use this zzounds affiliate link if you plan on going through with this. They'll match the lowest price + free shipping and no tax (unless you're in NJ).