Qt 4.8.3 is said to include better support for Apple Mac OS X 10.8 as well as QNX. There's also a new WebKit release (v2.2.3) and over "200 improvements" as part of this latest point release. The full change-log for Qt 4.8.3 can be found from the changes file, although obviously most of the work for this new Qt4 release comes down to just fixes.
There's also a blog post by Tuukka Turunen, the Director Qt R&D at Digia, announcing Qt 4.8.3. "Most of the activities have lately been around Qt 5, but rest assured – Qt 4.8 is still very much alive as well! ...As agreed with the Qt Project, Digia has packaged and tested Qt 4.8.3, which feeds from the Qt Project 4.8 branch. We will keep providing patch releases to 4.8 as long as needed. Currently we believe that at least two more patch releases will be needed. Qt 5 provides high compatibility with Qt 4 and we recommend Qt users to migrate their active projects to Qt 5.0 after the final version release is available."