As of today, the main difference that could impact your migration is the inclusion of MEF. So the difficulty of that task might depend on whether you want to adopt it or stay with Unity.
However, there might be new features and changes as the development continues, since it is iterational. The extent of your migration efforts will depend mostly on how many of these new features from Prism v4 you are going to include in your application.
Perhaps you could find this thread useful.
Another possibility, as you mentioned, is to start developing with Prism v4. Take into account that Prism v4 is currently in alpha state, and if you chose to do so, you would have to adapt your application as new drops appear.
As for the dates, there won’t be a Beta or RC. Instead, there will be biweekly drops that will be released in Codeplex. You can download the first drop
here. Additionally, there is an estimated release date in the blog post I mentioned.
I hope you find this helpful.
Guido Leandro Maliandi