Hi Sab,
Adobe Sensei is not a feature which can be integrated into a product. But it's the technology powering various features throughout Adobe products.
that means:
1) Some features powered by Adobe Sensei are available in AEM 6.4, e.g. Smart Crop.
2) At the moment I am not aware of that. The best way I could imagine is through the adobe.io platform (https://www.adobe.io)
3) What's the difference? :-)
4) sorry, I don't know.
kind regards,
Jörg