Now that Qt 5.0 is released, we can expect to see a number of questions popping up on the site relating to the new release.
We already have a tag: qt5
I'd say keep them separate - at least for now. We have versioning for many tags, to keep the questions - and more importantly, the sollutions - apart.
I think this is very helpful, especially in regards to APIs and considerably new versions, as it may or may not be consistent through the versions.
Merge 4 to 6 months after release, perhaps?