Moreover upstream recommended keeping Qt4 until 2017. If we get a Debian release every ±2 years that will make Jessie oldstable in 2017 and deprecated in 2018. This means we should really consider starting to port code using Qt4 to Qt5 during Stretch's developing life cycle.
It is important to note that Qt4 depends on a number of dependencies that their maintainers might want to get removed from the archive for similar reasons. In this case we will simply don't hesitate in removing their support as long as Qt4 keeps building. This normally doesn't mean API/ABI breakage but missing plugins that will diminish functionality from your apps, maybe even key ones. As an example let's take the **hypothetical** case in which the libasound2 maintainers are switching to a new libasound3 which is not API-compatible and removing libasound2 in the process. In this case we will have no choice but to remove the dependency and drop the functionality it provides. This is another of the important reasons why you should be switching to Qt5.
Qt4's webkit removalWebkit is definitely not an easy piece of code to maintain. For starters it means having a full copy of the code in the archive for both Qt4 and Qt5. Now add to that the fact that the code evolves quickly and thus having upstream support even for security bugs will be getting harder and harder. So we decided to remove Qt4's webkit from the archive. Of course we still have a lot of KDE stuff using Qt4's webkit, so it won't disappear "soon", but it will at some point.
PortingSome of us where involved in various Qt4 to Qt5 migrations  and we know for sure that porting stuff from Qt4 to Qt5 is much much easier and less painful than it was from Qt3 to Qt4.
We also understand that there is still a lot of software still using Qt4. In order to ease the transition time we have provided Wheezy backports for Qt5.
Don't forget to take a look at the C++ API changes page  whenever you start porting your application.
Temporarily shipping both Qt4 and Qt5 builds of your libraryIn case you maintain a library chances are that upstream already provides a way to build it using Qt5. Please note there is no point in shipping an application built with both flavours, please use Qt5 whenever possible. This double compilation should be left only for libraries.
You can't mix Qt4 and Qt5 in the same binary, but you may provide libraries compiled against one or the other. For example, your source package foo could provide both libqt4foo1 and libqt5foo1. You need to mangle your debian/rules and/or build system accordingly to achieve this.
A good example both for upstream code allowing both styles of compilation and debian packaging is phonon. Take a look at the CMakeLists.txt files for seeing how a source can be built against both flavours and another to debian/rules to see an example of how to handle the compilation. Just bear in mind that you
need to replace $(overridden_command) with the command itself, that variable substitution comes from internal stuff from our team and you should not be using it without a very good reason. If in doubt, feel free to ask us on IRC  or on the mailing list .
 irc.debian.org #debian-kde