1. Is Gantry 4 "natively" Responsive and Gantry 3 not? What is the difference?
Gantry for itself is a Template Framework, so for a responsive design it doesn't matter if it is Gantry 3 or Gantry 4. The Framework has nothing to do with the Gantry Template, they only have Gantry in their name.
The Gantry 3 Template on the other hand is NOT responsive. But thats no problem for you because you use Kirigami.
2. Is Kirigami "equally compatible" with Gantry 3 and Gantry 4?
Kirigami has responsive Design, so if that is your criteria Gantry 4 Template would be it´s counterpart.
In my personal opinion is Kirigami better because of the Typography
3. Are "Gantry 4 extensions" required or just preferable for full Gantry 4 feature sets?
Most extensions don't need Gantry Framework or Template to function properly.