We haven't yet released PT4 pricing at this time. If you purchase PT3 now you will definitely have PT4 at the price of PT3.
Once PT4 is available you will not need to use PT3 (in fact if you purchase PT3 now you don't need to use PT3 now either) and you will be able to convert your Holdem Manager database directly to PT4.
We will have a free beta period for PT4 that is open to everyone in the new year.