I'm sure someone's going to point out that Qt 4.8.3 was finalised last Autumn and that 4.8.4 was also done in 2012. These things take time to get adopted and out into the wild though, so it doesn't make the release note segments quoted below any less interesting.
From the release notes for Qt 4.8.3:
Qt for Symbian
- Fix Symbian key event code mapping in QSymbianControl [ou1cimx1#979068]
- On Symbian, use 16bit OpenGL if not enough GPU memory for 32bit [ou1cimx1#997217]
- Symbian: fix AlignedBlockAllocator initialization on emulator [ou1cimx1#998546]
- Symbian: inputcontext reports pointer events to FEP during inline edit [ou1cimx1#991638]
- Symbian: disable OpenVG as default on Symbian emulator builds [ou1cimx1#1005008]
- Symbian: create stub sis during build
- Removing potential crashes from out-of-thread cleanup on Symbian
- Enable smiley support for Symbian editors
- Fix for enter key inserting into password fields in Symbian
From the release notes for Qt 4.8.4:
Qt for Symbian
- Move SRCDIR definition to .pro file
- Fix symbian QtCore deffile (for QObjectPrivate)
- Fix symbian QtCore deffiles (for QUrl::detach)
- Handle Qt input hints for email and url editors
In addition to the quotes above, there are many platform-generic fixes and improvements, which Symbian phones getting the new Qt runtimes will also acquire.
Fingers crossed, eh? In case you're wondering, Belle Refresh devices currently ship with Qt 4.8.0 and Belle FP2 devices with Qt 4.8.1.
Any Qt developers reading this, perhaps you could comment on how likely these versions are to get used and out into mainstream distribution in the Symbian world?