You can't add API into Qt 4, there are new additions only in Qt 5 and as someone already explained, a lot of stuff from KDE widgets is being upstreamed to Qt 5. New widgets and API are being added. There are no or just little changes simply for one reason: why change something that works? To change stuff just for the sake of change is not a justification and it would only make it more difficult for devs to port their apps from Qt 4 to Qt 5.
