bluelines:

My bet is on the 9A1 engine, still 3.8l, bumped up to 400-420hp and with a particulate filter to meet the stricter emission requirements.

I don't think it has been increased to 4.0l because there is no need to. Output is expected around 400hp, not 500hp. I also don't think it will use the GT3 engine. Porsche would need to increase the price too much for the car to make sense. 

And raising the price is not that unusual for Porsche especially as a way to keep performance and value in line. If going my N/A it makes more sense to use the gt motor rather than keep two N/A versions running. More gt motors reduces cost of them and let’s face it paying $20 k more for the gt4 over the last one would still be a bargain with the gt motor. They would not be able to make enough.