4.0 is an exciting next step for Processing in which the internals of the software will see important updates, helping prepare the platform for its future. This includes the move to JDK 11 and support for new Java language features. The changes should be transparent to most users, but because of the massive shift behind the scenes, this will be 4.0.

As with all releases, we'll do everything possible to avoid breaking API. However, there will still be tweaks that have to be made. We'll try to keep them minor. Our goal is stability, and keeping everyone's code running.

• The static versions of selectInput(), selectOutput(), and selectFolder() in PApplet` have been removed. These were not documented, hopefully were not in use anywhere.
• The frame object has been removed from PApplet. We've been warning folks to use surface since 2015, but we still should warn users
• PImage.checkAlpha() is now public instead of protected
• All AWT calls have been moved out of PImage, which may be a problem for anything that was relying on those internals
• Removed MouseEvent.getClickCount() and MouseEvent.getAmount(). These had been deprecated, not clear they were used anywhere.

• Base.defaultFileMenu is now protected instead of static public