Qt3 to Qt4 required you to change your code to support Qt4.
There will also be porting needed for Qt4 to Qt5 (e.g. QtWidgets being a separate module, changing QtGui include to QtWidgets). See http://qt-project.org/wiki/Transition_from_Qt_4.x_to_Qt5 for the full list of changes.
I used QAudioFormat from QtMultimedia in a project I was developing, This got moved from the QtMultimedia package to the qtmobility-dev package and from the QtMultimedia folder to the QtMultimediaKit folder.
Are you going to use the Qt Q... variants or the KDE K... variants of the different classes? Oh, and you cannot use them interchangeably (e.g. the file open/save dialogs).
Also, there is more emphasis in Qt on the QtQuick platform which is another moving target. Even then, are you going to use pure QtQuick, or use the new Ubuntu Phone/Tablet APIs they are creating?
How about pizza vs chips [http://www.google.com/trends/explore?q=android%2C+linux#q=pizza%2C%20%20chips&cmpt=q]. The search for "chips" is flat and a lot lower than the growing trend for pizza.
Conclusion: no one likes chips anymore, so all chip shops, McDonalds, etc. are going to die out!
These Google trends are only part of the picture.
Once you have found what you are looking for (e.g. Qt or Gtk+ documentation), you are likely to bookmark it and go directly to it, so your search result will only apply once even if you visit the Gtk+ docs a lot. Also, if you have downloaded and installed the Gtk+/Qt/KDE documentation, you don't need to search Google to find it!
Q: What are the Debian, et. al. popularity contest stats for the Gtk+, Qt and KDE libraries? What about the dev packages? Documentation packages? Applications making use of these libraries?
A user is not likely to search for Gtk+ or Qt (and maybe not even KDE or GNOME). They are more likely to search for things like "Amarok", "Natilus" or even more likely things like "PDF viewer for Linux". Even then, if they are looking for a specific application they are likely to go through something like the Ubuntu Software Center, or follow the instructions to install a cool new app from a blog/review site like http://www.webupd8.org./
A developer is also not likely to perform a generic search for the UI framework or desktop environment either. They will either search for a specific query mentioning APIs or packages such as "How do I set the value of a GtkProgressBar?", "gtk_label_set_text HTML styling" or "libqt5-gui". Even then, they are likely to try sites like stackoverflow first. They are also likely to ask questions on the relevant forums, IRC channels, etc.
Q: What are the search/question trends for gtk+/gnome and qt/kde tagged questions on stackoverflow?
Q: What are the visitor stats like and comment counts like on the various Qt/KDE/Gtk+/GNOME pages, blogs and wikis (e.g. Allen Day's GNOME blog).
Q: What are the trends for the number of commits and developers to the Gtk+/GNOME/KDE/Qt source code repositories?
"There's no central method to search an email at this time with the way our records are set up, unfortunately," NSA Freedom of Information Act officer Cindy Blacker told me last week. The system is “a little antiquated and archaic," she added.
Maybe a little extra could be included in the next NSA budget for an Outlook license?
"Spock, did you see the looks on their faces?" "Yes, Captain, a sort of vacant contentment."