Here is the un-official policy - they are not 100% sure on the update path!
you get a free update to PT-IV is you purchased Protools III Core (PCI or Nubus) or Protools Project (PCI or Nubus) after June 1. If you purchased before June 1 you have to pay for the update ( price TBA)
If you have purchased PowerMix version of Protools after Oct 1 you will get a free update. If you purchased before Oct 1 it is also a paid update (price TBA)