Extjs 4.x.x was released more than two years ago, however, all released versions 4.0.x, 4.1.x and 4.2.x have critical bugs.
As example: a critical bug EXTJSIV-9229 in 4.2.0 http://www.sencha.com/forum/showthread.php?259453 This bug is still in 4.2.1 beta, but it is not in 4.1.1a and 3.x.x. This is about remote tree node expand and I believe it is commonly used.
Another difficult bug EXTJSIV-8231, in 4.1.1a as well as 4.2.0, (not in 3.x.x), http://www.sencha.com/forum/showthread.php?253837 . No workaround so far even I opened a ticket in the support portal
As a software provider, we cannot afford to release any product with such kind of critical bugs in any applications.