As it has not been released, we can only speculate.
Yes, you probably have to pay for it, as it will be a brand new release.
It may, in all probability, be able to be installed as a separate install, next to v3, as it is not an update of v3.
Why do you expect it to be free? You do not get Office 2016 for free, if you already have Office 2013, as an example.
Having said all that, the above is all speculation.
The best people, probably, to answer any P3D queries, would be within the Prepar3d forums on the Prepar3d site.