If you currently own Kontakt 1-3 (or a variety of other Native Instruments products) you can upgrade to Kontakt 4 for $74.50 until June 30. No reason to not jump on this now if you are still running K2. It comes with a prefty hefty 43GB sample library, so that alone is worth the price of the upgrade.
FYI - If you do this and are running Sibelius, you'll want to make sure that you're upgraded to Sibelius 6.2 or Sibelius won't be able to load Kontakt as a plugin. Haven't tested it in Sibelius 5, but I'd be willing to bet that you might have issues.