Your will be able to use your PT4 license when the Mac version is released. We don't yet have a date for this but we're hoping to have the first beta available before the end of the year. These things are hard to be predict though, so that is not yet set in stone.
In the meantime you're free to run PT4 using Parallels.
If you want to do this the best thing to do is to use the PostgreSQL service that you already have installed on your Mac - that will give you the best performance, and also allow PT4 to connect to your PT3 database to transfer the data across.
Check out
this guide which explains the simple changes you need to make to use PostgreSQL that way.
Nothing you do with PT4 will affect your PT3 database - you can continue to use that independently as long as you wish.