Qt-4.8.4 uses QtWebkit-2.2. Qt5 updated to 2.3, which brings that multithreaded rendering (and MANY fixes). There were discussions about a seperate QtWebkit-package, that could bring 2.3 to Qt-4.8*, but I never read about it again.
That would be a great option for Qt5, too, because WebKit is the critical part for security of the browser, so it really should get updates whenever there was a security issue. Currently QtWebkit only get's updates with Qt-updates, which is only one in several months.
Qt 5 has QtWebKit 3.0 I believe, QtWebKit 2.3 is specifically for Qt 4.8, but I haven't packaged it yet (was waiting for Qt 5.0 to release so it can follow the same branch), but you can get it as a git checkout.