PyQt4 is stable here, and I am afraid that Qt3 apps will never be compatible with Qt4 ones, that is, Qt4 will not include backward compatibility with Qt3. The developers will have to port their apps to Qt4 if they want. Qt3 and Qt4 can live together very well though, but not the Python bindings as far as I know, I have been looking at this and I didn't find a way to make them work together, and worst, nobody seems to be worried about it. The good thing in all this seems to be that there is not many apps written for PyQt, at least in our repo.
EDIT: Well I did a search in the repo and looks like we have 0
apps depending on PyQt3, that means that the risk is also 0