There will be no updates to version 3.5. Like version 2 it is a stand alone product. Version 4 will also be a stand alone. You pay for this version and get a product that does not expire.
As for version 4, who knows? The creators are working on it. They made the choice to do this in silence. Which I can understand: they don't want to make promises, give time frames, work in their own tempo, to not create any expectations. This is also part why they are not on Patreon for example.
Seeing the quality of 3.5, I am sure version 4 will be great. It is a unique piece of work, with all custom code and as far as I am concerned, they can take all the time in the world
