The new Tapatalk 4 will be merged with the existing "Tapatalk HD" when it comes out of beta testing. If you bought the HD version already, you don't need to purchase again. However, if you purchased the Taptalk 2.xx release, then you'll need to purchase the version 4.0. Though I'm certain you'll be entitled to a discount as it was the case with Tapatalk HD's release.
Hope that answers your question armada.