Not much was said about GTK4, since it will still be a ways off, and there isn't even any road-map or defined plans for that (or for when GNOME 4.0 could be here), but it was basically exposing what GTK still lacks and long-term areas that it could be improved upon.
After talking about how GTK3 is an improvement due to its dealing with Cairo for drawing, support for dynamic GDK back-ends, CSS theming, and various other enhancements, Benjamin Otte and Matthias Clasen talked about the future.
No one has started any work on GTK4 yet, considering it will be a ways off, but the current thinking is that the next tool-kit will improve in areas of animations, effects, and transformations. There will also be no GDK windows and there will be support for event bubbling.
Also brought up for GTK4 is re-thinking multi-platform support. GTK is supported on Mac OS X and Windows, but there is few active developers working on these platforms (there isn't even an official GTK3 Windows installer right now), and the support can be hit-or-miss. It needs to be thoroughly discussed what to do with this multi-platform GTK support. It wasn't ruled out to drop the support, but the likelihood is focusing upon how to improve the support with automated testing and finding developers to focus upon these non-Linux environments.
There was also talk of greater interoperability between the GTK and Clutter tool-kits.
Widget improvements and some other items were also talked about, but the items above were the most prominent focus of the half-hour talk.
It will be quite a while before likely seeing GTK+ v4.0, but at least it's already being talked about in the most early stages.